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Chemistry

Chemistry

Chemistry (derived from the Arabic word kimia, alchemy, where al is Arabic for the) is the science of matter that deals with the composition, structure, and properties of substances and with the transformations that they undergo. In the study of matter, chemistry also investigates its interactions with energy and itself (see physics, biology). Because of the diversity of matter, which is mostly composed of different combinations of atoms, chemists often study how atoms of different chemical elements interact to form molecules and how molecules interact with each other. molecules

Introduction

Chemistry is a large field encompassing many subdisciplines that often overlap with significant portions of other sciences. The fundamental component of chemistry is that it involves matter in some way (this explains its broad reach). It may involve the interaction of matter with non-material phenomena such as energy. More central to chemistry is the interaction of matter with other matter such as in the classic chemical reaction where chemical bonds are broken and made, forming new molecules. Matter, such as the chair you are sitting on or the air you breathe, is known today to consist of molecules. Each molecule consists of small bits of matter known as atoms that are connected together through chemical bonds. Each atom consists of smaller bits of matter known as subatomic particles. The structure of the world we commonly experience and the properties of the matter we commonly interact with are determined by the nature of this matter on the chemical level. Steel is hard because of how the atoms are bound together. Wood will burn because it can react with oxygen in a chemical reaction. Water is a liquid at room temperature because of how each molecule of water interacts with its neighbors. In fact, you are a thinking, sentient being because of an on-going series of chemical reactions and other chemical interactions. You can see this text because of how light interacts with molecules called proteins in the back of your eye. Chemistry is often called the central science because it is what connects most of the other sciences together. Chemistry is in some ways physics on a larger scale and in some ways is biology or geology on a smaller scale. Chemistry is used to understand and make better materials for engineering. It is used to understand the chemical mechanisms of disease as well as to create pharmaceuticals to treat disease. Chemistry is somehow involved in almost every science, every technology and every "thing". With such a large area of study, it is impossible to know everything about chemistry and very difficult to summarize the field concisely. Even the most knowledgable, experienced chemist only knows a very narrow area of chemistry better than others. Of course, most chemists have a broad general knowledge of many areas of chemistry as well. Chemistry is divided into many areas of study called subdisciplines in which chemists specialize. The chemistry taught at the high school or early college level is often called "general chemistry" and is intended to be an introduction to a wide variety of fundamental concepts and to give the student the tools to continue on to more advanced subjects. Many concepts presented at this level are often incomplete and technically inaccurate yet of extraordinary utility. Chemists regularly use these simple, elegant tools and explanations in their work when they suffice because the best solution possible is often so overwhelmingly difficult and the true solution is usually unobtainable. The science of chemistry is historically a recent development but has its roots in alchemy which has been practiced for millennia throughout the world. The word chemistry is directly derived from the word alchemy, however the etymology of alchemy is unclear (see alchemy).

Subdisciplines of chemistry

Chemistry typically is divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry. ; Analytical chemistry : Analytical chemistry is the analysis of material samples to gain an understanding of their chemical composition and structure. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdiciplines of chemistry, exluding purely theoretical chemistry. ; Biochemistry : Biochemistry is the study of the chemicals, chemical reactions and chemical interactions that take place in living organisms. Biochemistry and organic chemistry are closely related f.e. in medicinal chemistry. ; Inorganic chemistry : Inorganic chemistry is the study of the properties and reactions of inorganic compounds. The distinction between organic and inorganic disciplines is not absolute and there is much overlap, most importantly in the sub-discipline of organometallic chemistry. ; Organic chemistry : Organic chemistry is the study of the structure, properties, composition, mechanisms, and reactions of organic compounds. ; Physical chemistry : Physical chemistry or physicochemistry is the study of the physical basis of chemical systems and processes. In particular, the energetics and dynamics of such systems and processes are of interest to physical chemists. Important areas of study include chemical thermodynamics, chemical kinetics, electrochemistry, statistical mechanics, and spectroscopy. Physical chemistry has large overlap with molecular physics. ; Theoretical chemistry : Theoretical chemistry is the study of chemistry via theoretical reasoning (usually within mathematics or physics). In particular the application of quantum mechanics to chemistry is called quantum chemistry. Since the end of the second world war, the development of computers has allowed a systematic development of computational chemistry, which is the art of developing and applying computer programs for solving chemical problems. Theoretical chemistry has large overlap with molecular physics. ; Other fields : Astrochemistry, Atmospheric chemistry, Chemical Engineering, Electrochemistry, Environmental chemistry, Geochemistry, History of chemistry, Materials science, Medicinal chemistry, Molecular Biology, Molecular genetics, Nuclear chemistry, Organometallic chemistry, Petrochemistry, Pharmacology, Photochemistry, Phytochemistry, Polymer chemistry, Supramolecular chemistry, Surface chemistry, and Thermochemistry.

Fundamental concepts

Nomenclature

Nomenclature refers to the system for naming chemical compounds. There are well-defined systems in place for naming chemical species. Organic compounds are named according to the organic nomenclature system. Inorganic compounds are named according to the inorganic nomenclature system. See also: IUPAC nomenclature

Atoms

Main article: Atom. An atom is a collection of matter consisting of a positively charged core (the nucleus) which contains protons and neutrons, and which maintains a number of electrons to balance the positive charge in the nucleus.

Elements

Main article: Chemical element. An element is a class of atoms which have the same number of protons in the nucleus. This number is known as the atomic number of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium. The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together. Lists of the elements by name, by symbol, and by atomic number are also available. See also: isotope

Compounds

Main article: Chemical compound A compound is a substance with a fixed ratio of chemical elements which determines the composition, and a particular organisation which determines chemical properties. For example, water is a compound containing hydrogen and oxygen in the ratio of two to one, with the Oxygen between the hydrogens, and an angle of 104.5° between them. Compounds are formed and interconverted by chemical reactions.

Molecules

Main article: Molecule. A molecule is the smallest indivisible portion of a pure compound that retains a set of unique chemical properties. A molecule consists of two or more atoms covalently bonded together.

Ions

Main article: Ion. An ion is a charged species, or an atom or a molecule that has lost or gained an electron. Positively charged cations (e.g. sodium cation Na⁺) and negatively charged anions (e.g. chloride Cl^-) can form neutral salts (e.g. sodium chloride NaCl). Examples of polyatomic ions that do not split up during acid-base reactions are hydroxide (OH^-), or phosphate (PO₄^3-).

Bonding

Main article: Chemical bond. A chemical bond is an interaction which holds together atoms in molecules or crystals. In many simple compounds, valence bond theory and the concept of oxidation number can be used to predict molecular structure and composition. Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes, valence bond theory fails and alternative approaches which are based on quantum chemistry, such as molecular orbital theory, are necessary.

States of matter

Main article: Phase (matter). A phase is a set of states of a chemical system that have similar bulk structural properties, over a range of conditions, such as pressure or temperature. Physical properties, such as density and refractive index tend to fall within values characteristic of the phase. The phase of matter is defined by the phase transition, which is when energy put into or taken out of the system goes into rearranging the structure of the system, instead of changing the bulk conditions. Sometimes the distinction between phases can be continuous instead of having a discrete boundary, in this case the matter is considered to be in a supercritical state. When three states meet based on the conditions, it is known as a triple point and since this is invariant, it is a convenient way to define a set of conditions. The most familiar examples of phases are solids, liquids, and gases. Less familiar phases include plasmas, Bose-Einstein condensates and fermionic condensates and the paramagnetic and ferromagnetic phases of magnetic materials. Even the familiar ice has many different phases, depending on the pressure and temperature of the system. While most familiar phases deal with three-dimensional systems, it is also possible to define analogs in two-dimensional systems, which is getting a lot of attention because of its relevance to biology.

Chemical reactions

Main article: Chemical reaction. Chemical reactions are transformations in the fine structure of molecules. Such reactions can result in molecules attaching to each other to form larger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds.

Quantum chemistry

Main article: Quantum chemistry. Quantum chemistry describes the behavior of matter at the molecular scale. It is, in principle, possible to describe all chemical systems using this theory. In practice, only the simplest chemical systems may realistically be investigated in purely quantum mechanical terms, and approximations must be made for most practical purposes (e.g., Hartree-Fock, post Hartree-Fock or Density functional theory, see computational chemistry for more details). Hence a detailed understanding of quantum mechanics is not necessary for most chemistry, as the important implications of the theory (principally the orbital approximation) can be understood and applied in simpler terms.

Laws

The most fundamental concept in chemistry is the law of conservation of mass, which states that there is no detectable change in the quantity of matter during an ordinary chemical reaction. Modern physics shows that it is actually energy that is conserved, and that energy and mass are related; a concept which becomes important in nuclear chemistry. Conservation of energy leads to the important concepts of equilibrium, thermodynamics, and kinetics. Further laws of chemistry elaborate on the law of conservation of mass. Joseph Proust's law of definite composition says that pure chemicals are composed of elements in a definite formulation; we now know that the structural arrangement of these elements is also important. Dalton's law of multiple proportions says that these chemicals will present themselves in proportions that are small whole numbers (i.e. 1:2 O:H in water); although in many systems (notably biomacromolecules and minerals) the ratios tend to require large numbers, and are frequently represented as a fraction. Such compounds are known as Non-Stoichiometric Compounds More modern laws of chemistry define the relationship between energy and transformations.
- In equilibrium, molecules exist in mixture defined by the transformations possible on the timescale of the equilibrium, and are in a ratio defined by the intrinsic energy of the molecules—the lower the intrinsic energy, the more abundant the molecule.
- Transforming one structure to another requires the input of energy to cross an energy barrier; this can come from the intrinsic energy of the molecules themselves, or from an external source which will generally accelerate transformations. The higher the energy barrier, the slower the transformation occurs.
- There is a hypothetical intermediate, or transition structure, that corresponds to the structure at the top of the energy barrier. The Hammond-Leffler Postulate states that this structure looks most similar to the product or starting material which has intrinsic energy closest to that of the energy barrier. Stabilizing this hypothetical intermediate through chemical interaction is one way to achieve catalysis.
- All chemical processes are reversible (law of microscopic reversibility) although some processes have such an energy bias, they are essentially irreversible.

History of chemistry

- Alchemy
- Discovery of the chemical elements
- History of chemistry
- Nobel Prize in chemistry
- Timeline of chemical element discovery

Etymology

Old French: alkemie; Arab al-kimia: the art of transformation. See also: alchemy

External links

- [http://www.allchemicals.info/ Chemical Glossary]
- [http://chem.sis.nlm.nih.gov/chemidplus/ Chemistry Information Database includes basic information and some toxicity]
- [http://www.chem.qmw.ac.uk/iupac/ IUPAC Nomenclature Home Page], see especially the "Gold Book" containing definitions of standard chemical terms
- [http://www.cci.ethz.ch/index.html Experiments] videos and photos of the techniques and results
- [http://physchem.ox.ac.uk/MSDS/ Material safety data sheets for a variety of chemicals]
- [http://www.flinnsci.com/search_MSDS.asp Material Safety Data Sheets]

Arabic language

The Arabic language (; , less formally, ) is the largest member of the Semitic branch of the Afro-Asiatic language family (classification: South Central Semitic) and is closely related to Hebrew and Aramaic. It is spoken throughout the Arab world and is widely studied and known throughout the Islamic world. Arabic has been a literary language since at least the 6th century and is the liturgical language of Islam.

Literary and Modern Standard Arabic

The term "Arabic" may refer either to literary Arabic, which no Arab speaks as a mother tongue, or Modern Standard Arabic or to the many spoken varieties of Arabic commonly called "colloquial Arabic." Arabs consider literary Arabic as the standard language and tend to view everything else as mere dialects. Literary Arabic, (Literally: "the most eloquent Arabic language" — ) refers both to the language of present-day media across North Africa and the Middle East and to the more archaic language of the Qur'an. (The expression media here includes most television and radio, and all written matter, including all books, newspapers, magazines, documents of every kind, and reading primers for small children.) "Colloquial" or "dialectal" Arabic refers to the many national or regional dialects/languages derived from Classical Arabic, spoken daily across North Africa and the Middle East, which constitute the everyday spoken language. These sometimes differ enough to be mutually incomprehensible. These dialects are not typically written, although a certain amount of literature (particularly plays and poetry) exists in many of them. They are often used to varying degrees in informal spoken media, such as soap operas and talk shows. Literary Arabic or classical Arabic, is the official language of all Arab countries and is the only form of Arabic taught in schools at all stages. The sociolinguistic situation of Arabic in modern times provides a prime example of the linguistic phenomenon of Diglossia -the normal use of two separate varieties of the same language, usually in different social situations. In the case of Arabic, educated Arabs of whatever nationality can be assumed to speak both their local dialect and their school-taught literary Arabic (to an equal or lesser degree). This diglossic situation facilitates code switching in which a speaker switches back and forth unaware between the two varieties of the language, sometimes even within the same sentence. In instances in which Arabs of different nationalities engage in conversation only to find their dialects mutually unintelligible (e.g. a Moroccan speaking with a Lebanese), both should be able to code switch into Literary Arabic for the sake of communication. Since the written Arabic of today differs from the written Arabic of the Qur'anic era, it has become customary in western scholarship and among non-Arab scholars of Arabic to refer to the language of the Qur'an as Classical Arabic and the modern language of the media and of formal speeches as Modern Standard Arabic. Arabs, on the other hand, often use the term to refer to both forms, thus placing greater emphasis on the similarities between the two. The difference between Arabic of the Qur'anic era and today's Classical Arabic is only in the degree of eloquance. The vocabulary, the syntatic and grammatical rules are the same. Quite a few English words are ultimately derived from Arabic, often through other European languages, especially Spanish, among them every-day vocabulary like sugar (sukkar), cotton (qutn) or magazine (). More recognizable are words like algorithm, algebra, alchemy, alcohol, azimuth, nadir, and zenith (see List of English words of Arabic origin). The Maltese language spoken on the Mediterranean island of Malta is the only surviving European language to derive primarily from Arabic (a North African dialect), though it contains a large number of Italian and English borrowings.

Arabic and Islam

It is sometimes difficult to translate Islamic concepts, and concepts specific to Arab culture, without using the original Arabic terminology. The Qur'an is expressed in Arabic and traditionally Muslims deem it impossible to translate in a way that would adequately reflect its exact meaning—indeed, until recently, some schools of thought maintained that it should not be translated at all. A list of Islamic terms in Arabic covers those terms which are too specific to translate in one phrase. While Arabic is strongly associated with Islam (and is the language of salah), it is also spoken by Arab Christians, Oriental (Sephardic) Jews, and smaller sects such as Iraqi Mandaeans. Even so, a majority of the world's Muslims do not actually speak Arabic, but only know some fixed phrases of Arabic, such as those used in Islamic prayer. However, to counteract this, there is great encouragement for non-Arabic-speaking Muslims to learn the language.

Dialects

See Varieties of Arabic for a fuller overview. "Colloquial Arabic" is a collective term for the spoken languages or dialects of people throughout the Arab world, which, as mentioned, differ radically from the literary language. The main dialectal division is between the Maghreb dialects and those of the Middle East, followed by that between sedentary dialects and the much more conservative Bedouin dialects. Maltese, though descended from Arabic, is considered a separate language. Speakers of some of these dialects are unable to converse with speakers of another dialect of Arabic; in particular, while Middle Easterners can generally understand one another, they often have trouble understanding Maghrebis (although the converse is not true, due to the popularity of Middle Eastern—especially Egyptian—films and other media). One factor in the differentiation of the dialects is influence from the languages previously spoken in the areas, which have typically provided a significant number of new words, and have sometimes also influenced pronunciation or word order; however, a much more significant factor for most dialects is, as among Romance languages, retention (or change of meaning) of different classical forms. Thus Iraqi aku, Levantine fiih, and North African kayen all mean "there is", and all come from Arabic (yakuun, fiihi, kaa'in respectively), but now sound very different. The major groups are:
- Egyptian Arabic (Egypt) Considered the most widely understood and used "second dialect"
- Maghreb Arabic (Algerian Arabic, Moroccan Arabic, Tunisian Arabic and western Libyan)
- Levantine Arabic (Western Syrian, Lebanese, Palestinian, and western Jordanian, Cypriot Maronite Arabic)
- Iraqi Arabic or Gulf Arabic (Iraqi, Eastern Syrian, Kuwaiti, Saudi Arabian, Persian Gulf coast from Iraq to Oman including much of Saudi Arabia's Eastern Province, and minorities on the other side) Other varieties include:
- (in Mauritania and Western Sahara)
- Andalusi Arabic (extinct, but important role in literary history)
- Maltese
- Sudanese Arabic (with a dialect continuum into Chad)
- Hijazi Arabic (West Cost of Saudi Arabia, Northern Saudi Arabia, eastern Jordan, Western Iraq)
- Najdi Arabic (Najd region of central Saudi Arabia)
- Yemeni Arabic (Yemen to southern Saudi Arabia)

Phonology

The consonant phonemes below reflect the pronunciation of Standard Arabic, which has only three vowels, in short and long variants, namely and . Naturally, considerable allophony occurs.

Consonants

Standard Arabic has 28 consonants: See Arabic alphabet for explanations on the IPA phonetic symbols found in this chart. # is pronounced as by some speakers. This is especially characteristic of the Egyptian and southern Yemeni dialects. In many parts of North Africa and in the Levant, it is pronounced as . # is pronounced only in , the name of God, i.e. Allah. # is usually a phonetic approximant. # In many varieties (if not most), are actually epiglottal (despite what is reported in many earlier works).

Emphatic Consonants

The consonants traditionally known as "emphatic" are either velarised or pharyngealised . In some transcription systems, emphasis is shown by capitalizing the letter e.g. is written ‹D›; in others the letter is underlined or has a dot below it e.g. ‹ḍ›.

Long Consonants

Vowels and consonants can be (phonologically) short or long. Long (geminate) consonants are normally written doubled in Latin transcription (i.e. bb, dd, etc.), reflecting the presence of the Arabic diacritic mark shaddah, which marks lengthened consonants. Such consonants are held twice as long as short consonants. This consonant lengthening is phonemically contrastive: e.g. qabala "he received" and qabbala "he kissed".

Syllable Shape

Arabic has two kinds of syllable: open syllables (CV) and (CVV) - and closed syllables (CVC), (CVVC) and (CVCC). Every syllable begins with a consonant - or else a consonant is borrowed from a previous word through elision – especially in the case of the definite article THE, al (used when starting an utterance) or _l (when following a word), e.g. baytu –l mudiir “house (of) the director”, which becomes bay-tul-mu-diir when divided syllabically. By itself, definite mudiir would be pronounced .

Word Stress

Although word stress is not phonemically contrastive in Standard Arabic, it does bear a strong relationship to vowel length and syllable shape, and correct word stress aids intelligibility. In general, "heavy" syllables attract stress (i.e. syllables of longer duration - a closed syllable or a syllable with a long vowel). In a word with a syllable with one long vowel, the long vowel attracts the stress (e.g. ki-'taab and ‘kaa-tib). In a word with two long vowels, the second long vowel attracts stress (e.g.ma-kaa-'tiib). In a word with a "heavy" syllable where two consonants occur together or the same consonant is doubled, the (last) heavy syllable attracts stress (e.g. ya-ma-’niyy, ka-'tabt, ka-‘tab-na, ma-‘jal-lah, ‘mad-ra-sah, yur-‘sil-na). This last rule trumps the first two: ja-zaa-i-‘riyy. Otherwise, word stress typically falls on the first syllable: ‘ya-man, ‘ka-ta-bat, etc. The Cairo (Egyptian Arabic) dialect, however, has some idiosyncrasies in that a heavy syllable may not carry stress more than two syllables from the end of a word, so that mad-‘ra-sah carries the stress on the second-to-last syllable, as does qaa-‘hi-rah.

Dialectical Phonologies

In some dialects, there may be more or fewer phonemes than those listed in the chart above. For example, non-Arabic is used in the Maghreb dialects as well in the written language mostly for foreign names. Semitic became extremely early on in Arabic before it was written down; a few modern Arabic dialects, such as Iraqi (influenced by Persian) distinguish between and . Interdental fricatives ( and ) are rendered as stops and in some dialects (principally Levantine and Egyptian) and as and in "learned" words from the Standard language. Early in the expansion of Arabic, the separate emphatic phonemes and coallesced into a single phoneme, becoming one or the other. Predictably, dialects without interdental fricatives use exclusively, while those with such fricatives use . Again, in "learned" words from the Standard language, is rendered as in dialects without interdental fricatives. Another key distinguishing mark of Arabic dialects is how they render Standard (a voiceless uvular stop): it retains its original pronunciation in widely scattered regions such as Yemen and Morocco (and among the Druze), while it is rendered in Gulf Arabic, Iraqi Arabic, Upper Egypt and less urban parts of the Levant (e.g. Jordan) and as a glottal stop in many prestige dialects, such as those spoken in Cairo, Beirut and Damascus. Thus, Arabs instantly give away their geographical (and class) origin by their pronunciation of a word such as qamar "moon": , or .

Grammar

See Arabic grammar

Alphabet

Arabic alphabet

Main article: Arabic alphabet The Arabic alphabet derives from the Aramaic script (which variety - Nabataean or Syriac - is a matter of scholarly dispute), to which it bears a loose resemblance like that of Coptic or Cyrillic script to Greek script. Traditionally, there were several differences between the Western (Maghrebi) and Eastern version of the alphabet—in particular, the fa and qaf had a dot underneath and a single dot above respectively in the Maghreb, and the order of the letters was slightly different (at least when they were used as numerals). However, the old Maghrebi variant has been abandoned except for calligraphic purposes in the Maghreb itself, and remains in use mainly in the Quranic schools (zaouias) of West Africa. Arabic, like other Semitic languages, is written from right to left.

Calligraphy

See Arabic calligraphy for a fuller overview. After the definitive fixing of the Arabic script around 786, by Khalil ibn Ahmad al Farahidi, many styles were developed, both for the writing down of the Qur'an and other books, and for inscriptions on monuments as decoration. Kufic font Arabic calligraphy has not fallen out of use as in the Western world, and is still considered by Arabs as a major art form; calligraphers are held in great esteem. Being cursive by nature, unlike the Latin alphabet, Arabic script is used to write down a verse of the Qur'an, a Hadith, or simply a proverb, in a spectacular composition. The composition is often abstract, but sometimes the writing is shaped into an actual form such as that of an animal. Two of the current masters of the genre are Hassan Massoudy and [http://arabworld.nitle.org/gallery.php?module_id=7 Khaled Al Saa’i].

Arabic using the Latin alphabet

See Arabic transliteration and Arabic Chat Alphabet for more information. There are a number of different standards of Arabic transliteration: methods of accurately and efficently representing Arabic with the Latin alphabet. The more scientific standards allow the reader to recreate the exact word using the Arabic alphabet. However, these systems are heavily reliant on diacritical marks, which may be difficult to pronounce at first sight. Other, less scientific, systems often use digraphs (like sh and kh), which are usually more simple to read, but sacrifice the definiteness of the scientific systems. During the last few decades and especially since the 1990s, Western-invented text communication technologies have become prevalent in the Arab world, such as personal computers, the World Wide Web, email, Bulletin board systems, IRC, instant messaging and mobile phone text messaging. Most of these technologies originally had the ability to communicate using the Latin alphabet only, and some of them still do not have the Arabic alphabet as an optional feature. As a result, Arabic speaking users communicated in these technologies by transliterating the Arabic text using the Latin script. To handle those Arabic letters that do not have an approximate equivalent in the Latin script, numerals and other characters were appropriated. E.g., the Latin numeral "3" is used to represent the Arabic letter "ع" ("ayn"). There is no universal name for this type of transliteration, but some have named it Arabic Chat Alphabet.

External links

- [http://arabic-media.com/ Arabic-Media] on-line access to Arabic newspapers, radio, and television
- [http://st-takla.org/Learn_Languages/01_Learn_Arabic-ta3leem-3araby/Learn-Arabic_00-index_El-Fehres.html Learn Arabic language online with audio pronunciation] from [http://St-Takla.org St. Takla Egyptian Church]
- [http://www.nicoweb.com/sirpus/learn%20arabic%20course%20mp3.htm Arabic Writing and Reading with MP3]. Arabic Writing and Reading Course Online with MP3 audio.
- [http://pince31.free.fr/lang/arabic/liens.htm Links to learn Arabic language with online course]
- [http://www.madinaharabic.com Arabic language learning course with audio]
- [http://www.dailystar.com.lb/article.asp?edition_id=10&categ_id=4&article_id=6173 "Antonyms in Arabic are a strange phenomenon" by Tamim al-Barghouti]
- [http://arabworld.nitle.org/texts.php?module_id=1&reading_id=17 "The Development of Classical Arabic" by Kees Versteegh]
- [http://arabworld.nitle.org/audiovisual.php?module_id=1&selected_feed=118 Wellesley College Professor of Arabic on the forms and dialects of the language]
- [http://www.uga.edu/islam/arabic_windows.html Multilingual Computing in Arabic with Windows, major word processors, web browsers, Arabic keyboards, and Arabic transliteration fonts]
- [http://www.gomideast.com/arabic/index.htm gomideast - Learning to Speak Arabic phrases]
- [http://language-directory.50webs.com/languages/arabic.htm List of online Arabic-related resources] Web references and examples:
- [http://transliteration.org/quran/Pronunciation/Letters/TashP.htm Arabic language pronunciation applet] with audio samples
- [http://www.sunna.info/teaching/ Learn Arabic]
- [http://www.everything2.com/index.pl?node_id=1289272 E2 article]
- [http://www.sprachprofi.de.vu/english/ar.htm Sprachprofi]
- [http://www.websters-online-dictionary.org/definition/Arabic-english/ Arabic - English Dictionary]: from [http://www.websters-online-dictionary.org Webster's Online Dictionary] - the Rosetta Edition.
- [http://www.ethnologue.com/show_language.asp?code=arb SIL's Ethnologue]
- [http://www.nitle.org/arabworld/texts.php?module_id=1&reading_id=113 Dialects of Arabic]
- [http://www.muftah-alhuruf.com Muftah-Alhuruf.com]: Write and send Arabic emails without having an Arabic keyboard or operating system. Arabic languages samples:
- [http://www.language-museum.com/a/arabic.php Arabic]
- [http://www.language-museum.com/a/arabic-chadian-spoken.php Arabic Chadian Spoken]
- [http://www.language-museum.com/a/arabic-judeo-iraqi.php Arabic Judeo Iraqi]
- [http://www.language-museum.com/a/arabic-north-levantine-spoken.php Arabic North Levantine Spoken]
- [http://arabworld.nitle.org/texts.php?module_id=1&reading_id=17 "The Development of Classical Arabic" by Kees Versteegh]
- Category:Arab ko:아랍어 ms:Bahasa Arab ja:アラビア語 simple:Arabic language th:ภาษาอาหรับ

Matter

Matter is commonly referred to as the substance of which physical objects are composed. In physics, it is everything that is constituted of elementary fermions. Philosophically, matter constitutes the formless substratum of all things, which exists only potentially and from which reality is produced. In the sense of content, matter is also used in contrast to form.

Matter in physics

Matter occupies space and has mass. It is composed predominantly of atoms, which consist of protons, neutrons, and electrons. All gauge bosons (of which the photon is one), which mediate the four fundamental forces, are not considered matter, even though they certainly have energy and some also mass. Matter thus consists of quarks and leptons. There are six types of quarks (strange, charm, top, bottom, up, and down) which combine to form hadrons, primarily baryons and mesons, through the strong interaction and are actually thought to always be confined. Among the baryons are the proton and the neutron, which further combine to form the nuclei of all elements of the periodic table. Usually these nuclei are surrounded by a cloud of electrons. A nucleus with as many electrons as protons, which is thus electrically neutral, is called an atom, otherwise it is an ion. Chemistry is the science that studies how nuclei and electrons combine to form compounds. In bulk, matter can exist in several different phases, according to particle density and energy density or alternatively pressure and temperature. These phases include gases, plasmas, liquids, fluids, superfluids, solids, and Bose-Einstein condensate. As circumstances change, matter may change from one phase into another. These phenomena are called phase transitions, and their energetics are studied in the field of thermodynamics. In small quantities, matter can exhibit properties that are entirely different from those of bulk material. Homogeneous matter has a definite composition and properties and any amount of the matter has the same composition and properties. Homogenous matter may or may not be a mixture. Iron and brass would examples of each. Heterogeneous matter does not have a definite composition, for example, granite. Matter constitutes the observable Universe. It can be converted to energy (see annihilation), and vice versa - can be created out of energy (see matter creation) and undergo other formations and alterations.

Chemical substance

A chemical substance is any material substance used in or obtained by a process in chemistry:
- A chemical compound is a substance consisting of two or more chemical elements that are chemically combined in fixed proportions.
- A chemical element is a substance that cannot be divided or changed into different substances by ordinary chemical methods. The smallest particle of such an element is an atom, which consists of electrons centered on a nucleus of protons and neutrons.
- A molecule is the smallest particle of an element or compound that retains the characteristics of the element or compound.
- An ion is an atom or group of atoms with a net electric charge, having lost (cation) or gained (anion) an electron.
- A chemical reaction is a process involving one, two or more substances (called reactants), characterized by a chemical change and yielding one or more product(s) which are different from the reactants. Category:Chemistry ja:薬品 th:สารเคมี

Chemical transformation

In chemistry a chemical transformation shows the conversion of a substrate to a product omitting the reagents and catalysts or underlying reaction mechanism as opposed to a chemical reaction. The phrase Chemical transformation is used in a more general sense when reaction specifics are not relevant or not available. The IUPAC Nomenclature for Transformations is based on this type of transformations. Category:Chemical processes Category:Chemical reactions

Matter

Matter in physics

Atom

:For alternative meanings see atom (disambiguation). An atom (Greek άτομον from ά: non and τομον: divisible) is a submicroscopic structure found in all ordinary matter. It is the smallest unit of an element to retain all the chemical properties of that element. The word atom originally meant a smallest possible particle of matter, not further divisible. Later, the objects that had been called atoms were found to be further divisible into smaller subatomic particles, but the word atom nonetheless continues to refer to them. Most atoms are composed of three types of massive subatomic particles which govern their external properties:
- electrons, which have a negative charge and are the least massive of the three;
- protons, which have a positive charge and are about 1836 times more massive than electrons; and
- neutrons, which have no charge and are about 1838 times more massive than electrons. Together, protons and neutrons form the nucleus of an atom, which is surrounded by the electrons. Atoms can differ in the number of each of the subatomic particles they contain. Atoms of the same element have the same number of protons, although the same element can differ in the number of neutrons which are then called isotopes of that element. Atoms are electrostatically neutral if they have an equal number of protons and electrons. Atoms which have either gained or lost electrons are called ions. Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. Atoms are able to bond into molecules and other types of chemical compounds. Molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen and one oxygen atom.

Properties of the atom

Subatomic particles

:see main article subatomic particles Up until 1961, the subatomic particles were thought to consist of only protons, neutrons and electrons. However, protons and neutrons themselves are now known to consist of varieties of a still smaller particle called the quark, and the electron is considered a type of lepton. Therefore in modern atomic theory, the two basic constituents of matter are the lepton and the quark of which the above three particles of the atom are composed. All particles exhibit a wave-particle duality so that the electron is better understood as a wave when drawn about a nucleus. Unlike planets revolving around the sun, the electron is not held around the nucleus of the atom by gravity, but rather by electromagnetism.
Atom sizes
The atom is many times smaller than the wavelength that human vision can detect in any kind of microscope. However, there are ways of projecting the atom so as to obtain amplified images of it. These include: scanning tunneling microscopy (STM), atomic force microscopy (ATM), and nuclear magnetic resonance (NMR). In measuring an atom, the size of the area that an electron can travel in must be determined. Electrons travel in areas called atomic orbitals. This area forms a cloud where the electron may be situated. In the helium atom above (shown in its ground state), the atomic orbital where the electron may be situated describes a sphere. However, the cloud or atomic orbital in which an electron can travel changes shapes depending on the energy of the electron. So some electrons travel in the shape of a dumbbell with the nucleus in the smallest space in-between. There are other more complicated shapes as well. And the heavier the element, the more electrons there are and the more shapes there are for the orbitals in the atom. It therefore not only becomes more complicated to measure the size of the atom, but it becomes complicated to create models of the atoms of heavier elements. Since the electron orbitals are considered clouds, then the size of an atom is not easily defined since the places where the electron can be just gradually go to zero as the distance from the nucleus increases. For atoms that can form solid crystals, the distance between adjacent nuclei can give an estimate of the atom size. For atoms that do not form solid crystals other techniques are used, including theoretical calculations. As an example, the size of a hydrogen atom is estimated to be approximately 1.0586×10 m. Compare this to the size of the proton which is the only particle in the nucleus of the hydrogen atom which is approximately 10 m. Thus the ratio of the sizes of the hydrogen atom to its nucleus is about 100,000:1. Atoms of different elements do vary in size, but the sizes are roughly the same to within a factor of 2 or so. The reason for this is that elements with a large positive charge on the nucleus attract the electrons to the center of the atom more strongly. To illustrate the size of an atom, one million atoms can fit within the breadth of a strand of hair. An atom is mostly space. A basic analogy for the ratio of space inside an atom is this: if an atom were the size of a baseball stadium, the nucleus would be the size of a marble on second base and the electrons would orbit the perimeter.
Elements, isotopes and ions
Atoms are generally classified by their atomic number, which corresponds to the number of protons in the atom. The atomic number defines which element the atom is. For example, carbon atoms are those atoms containing six protons. All atoms with the same atomic number share a wide variety of physical properties and exhibit the same chemical behavior. The various kinds of atoms are listed in the periodic table in order of increasing atomic number. The mass number, atomic mass number, or nucleon number of an element is the total number of protons and neutrons in an atom of that element, because each proton or neutron essentially has a mass of 1 amu. The number of neutrons in an atom has no effect on which element it is. Each element can have numerous different atoms with the same number of protons and electrons, but varying numbers of neutrons. Each has the same atomic number but a different mass number. These are called the isotopes of an element. When writing the name of an isotope, the element name is followed by the mass number. For example, carbon-14 contains 6 protons and 8 neutrons in each atom, for a total mass number of 14. The simplest atom is the hydrogen atom, which has atomic number 1 and consists of one proton and one electron. The hydrogen isotope which also contains one neutron so is called deuterium or hydrogen-2; the hydrogen isotope with two neutrons is called tritium or hydrogen-3. Tritium is an unstable isotope which causes the atom to lose mass in a process called radioactivity. The elements in the periodic table beginning with number 86, radon, and those that follow have no stable isotopes and are all radioactive. The atomic mass listed for each element in the periodic table is an average of the isotope masses found in nature, weighted by their abundance. Although most sources state that there are 92 elements that occur naturally on earth from hydrogen up to uranium in the periodic table, it has been recently discovered that plutonium, the 94th element, also occurs naturally. Most of these elements were created through stellar nucleosynthesis and supernova nucleosynthesis. Several elements that do not occur on earth have been found to be present in stars. Elements not normally found in nature have been artificially created by nuclear bombardment, but they are usually unstable and spontaneously change into stable natural chemical elements by the processes of radioactive decay. Atoms that have either lost or gained electrons are called atomic ions (with either positive(+) or negative charge(−), respectively). Atoms are canonically distinguished from ions by their balanced electrical charge.
Atomic spectrum
:see main article Atomic spectroscopy Each element in the periodic table therefore consists of an atom in a unique configuration i.e. with different amounts of protons in the nucleus. Each atom of each element can also be uniquely described by the shapes of its atomic orbitals and the number of electrons within them. There is also another way in which each element with its own configuration is distinctive, that is, by its atomic spectrum. A spectrum is created when light is passed through a prism and the light breaks up into its component colors. Spectroscopy studies the spectrum of each element. Each atom of each element creates its own light pattern unique to itself, its own spectral signature. Scientists can use a spectrometer to study the atoms in stars and other distant objects, and due to the distinctive spectral lines that each element produces, are able to tell the chemical composition of distant planets, stars and galaxies.
Electron configuration
:see main article electron configuration The chemical behavior of atoms is largely due to interactions between electrons. Electrons of an atom remain within certain, predictable electron configurations. Electrons fall into shells based on their relative energy level. Generally, the higher the energy level of a shell, the further away it is from the nucleus. The electrons in the outermost shell, called the valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus. valence electrons of a hydrogen atom. The principal quantum number is at the right of each row and the azimuthal quantum number is denoted by letter at top of each column.]] An electron shell can hold up to 2n² electrons, where n is the number of the shell. Whichever occupied shell is currently most outward is the valence shell, even if it only has one electron. In the most stable state, an atom's electrons will fill up its shells in order of increasing energy. Under some circumstances an electron may be excited to a higher energy level (that is, it absorbs energy from an external source and leaps to a higher shell), leaving a space in a lower shell, but at some point it will fall back to its previous level, emitting its excess energy as a photon. Electron shells also have distinctive shapes denoted by letters. In the illustration, the letters s, p, and d describe the shape of the atomic orbital. Electrons also have another property that describes their configuration due to the fact that they rotate in space. Thus electrons are said to have spin (physics).
Valence and bonding
:see main article valence electrons and chemical bond The number of electrons in an atom's outermost shell (ie the valence shell) governs its bonding behavior. Therefore, elements with the same number of valence electrons are grouped together in the periodic table of the elements. Group (i.e. column) 1 elements contain one electron on their outer shell; Group 2, two electrons; Group 3, three electrons; etc. As a general rule, the fewer electrons in an atom's valence shell, the more reactive it is. Group 1 metals are therefore very reactive, with caesium, rubidium, and francium being the most reactive of all metals. Every atom is much more stable (i.e. less energetic) with a full valence shell. This can be achieved one of two ways: an atom can either share electrons with neighboring atoms (a covalent bond), or it can remove electrons from other atoms (an ionic bond). Another form of ionic bonding involves an atom giving some of its electrons to another atom; this also works because it can end up with a full valence by giving up its entire outer shell. By moving electrons, the two atoms become linked. This is known as chemical bonding and serves to build atoms into molecules or ionic compounds. Five major types of bonds exist:
- ionic bonds;
- covalent bonds;
- coordinate covalent bonds;
- hydrogen bonds; and
- metallic bonds.
Atoms and antimatter
:see main article antimatter Antimatter can also form atoms, composed of antielectrons (positrons), antiprotons, and antineutrons.

Atoms and the Big Bang

In models of the Big Bang, Big Bang nucleosynthesis predicts that within one to three minutes of the Big Bang all the current atomic material in the universe was created producing no heavier element than lithium, but mostly hydrogen and helium. However, although the basic atomic particles of matter were created, atoms themselves could not form in the intense heat. Big Bang chronology of the atom continues to approximately 379,000 years after the Big Bang when the cosmic temperature had dropped to just 3,000 K which allowed the first atoms to form. It was then cool enough to allow protons to capture one electron each and form neutral atoms of hydrogen. Hydrogen makes up approximately 75% of the atoms in the universe. Helium makes up 24% and all other elements make up 1%. Since the size of the universe is unknown, the total numbers of atoms in the universe is unknown, but the number is not thought to be infinite because current theory suggests we live in a finite universe. One thing we can say about the mass of the baryons in the universe, meaning the mass of the protons and neutrons, is that we can tell what the ratio of their density ought to be from the Big Bang model. Einstein's theory of General Relativity suggests that the universe is the same in all directions and from all viewpoints. Therefore, examining one region of the universe and the density of atoms in that region should tell us how densely atoms are scattered throughout the entire universe, but as said previously, does not tell us how far the universe extends and how many atoms exist in total. Big Bang Nucleosynthesis predicts that 1/20 of the total mass of the Universe is baryonic matter. (The baryon is the category used to describe neutrons and protons which are similar in mass but different in electric charge.) So theoretically we should be able to study a region of space and calculate the amount of matter we see through our telescopes and one-twentieth of the matter should be baryons. However, from the density we can see through telescopes of matter in regions of the visible universe, 99% of the baryons are missing. This has given rise to theories of dark matter (which should also be made of baryons--or if you prefer atoms, since baryons make up the nucleus of atoms) in order to make up the difference in missing matter. What that means is that there are probably more atoms out there than we can see through our usual means of detection. In other words, we cannot see visible light from these atoms nor have we detected electromagnetic radiation, but they exist. In fact, in some cases we have detected, through radio-wave detectors, entire galaxies such as Virgo H121 that do not appear in normal telescopes.

Atomic theory

The atomic theory is a theory of the nature of matter. It states that all matter is composed of atoms.

Historical theories

Democritus and Leucippus, Greek philosophers in the 5th century BC, presented the first theory of atoms (see article atomism for more details). They held that each atom had a different shape, like a pebble, that governed the atom's properties. Dalton and Avogadro rediscovered the works of Democritus and Leucippus and suggested in the 19th century that matter was made up of atoms, but they knew nothing of their structure. This theory was conflicting with the theory of infinite divisibility, which states that matter can always be divided into smaller parts. The controversy ended in 1911 when Jean Perrin demonstrated the existence of atoms through experimental validation of Einstein's theory of Brownian motion (which relied on atomic theory). For much of this time, atoms were thought to be the smallest possible piece of matter. However, in 1897, J.J. Thomson published his work proving that cathode rays are made of negatively charged particles (electrons). Since cathode rays are essentially emitted from matter, this proved that atoms are made up of subatomic particles and are therefore divisible, and not the indivisible "atomos" Democritus talked about. Physicists later invented a new term for indivisible units, namely elementary particles since the word atom had already been taken and come into common use. At first, it was believed that the electrons were distributed more or less uniformly in a sea of positive charge (the plum pudding model). However, an experiment conducted a few years later by Rutherford demonstrated that atoms are mostly empty space, with a lot of mass concentrated in a nucleus. In the gold foil experiment, he shot alpha particles (emitted by polonium) through a sheet of gold. He observed that most of the particles passed straight through the sheet without deflection (striking a fluorescent screen on the other side), but that, surprisingly, a small number were bounced right back (having come close to a nucleus). This led to the planetary model of the atom, in which the electrons orbited the nucleus like the planets orbiting the sun. The nucleus was later discovered to contain protons, and further experimentation by Rutherford found that the nuclear mass of most atoms surpassed the number of protons it possessed; this led him to postulate the existence of neutrons, whose existence would be proven in 1932 by James Chadwick. The planetary model of the atom still had shortcomings. Firstly, a moving electrical charge emits electromagnetic waves; according to classical physics, an orbiting charge would steadily lose energy and spiral towards the nucleus, eventually colliding with it. Secondly, the model didn't explain why hydrogen gas, when submitted to an electrical discharge, emitted light only in certain discrete spectra. Experiments by Max Planck and Albert Einstein demonstrated that energy is transferred in tiny fixed amounts known as quanta. In 1913, Niels Bohr used this idea in his Bohr model of the atom, in which the electrons could only orbit the nucleus in fixed circles. They couldn't spiral downwards because they couldn't lose energy in a continuous manner; they could only make quantum leaps between fixed energy levels. The Bohr model would eventually be replaced by a full quantum mechanics model in 1925.

Study of atoms

Because of their ubiquitous nature, atoms have been an important field of study for many centuries. Current research focuses on quantum effects, such as in Bose-Einstein condensate. The study of atoms was done largely by indirect means through the 19th century and early 20th century. In recent years, however, new techniques have made the identification and study of atoms easier and more accurate. The electron microscope, invented in 1931, can image large molecules, however, not the atom itself. Atomic force microscopy is another technique by which individual atoms can be visualized and even arranged into patterns. Methods also exist to identify atoms and compounds. Elemental analysis allows the exact identification of the types and amounts of atoms in a substance.

Practical uses of the atom

Atoms have given us the key to understanding our universe, understanding our earth and life upon it, improving technology, and creating life-saving pharmaceuticals. There does not exist a scientific field that is not affected by the understanding of the atom. Atoms are the basis for chemistry, physics, geology, astronomy and biology. Within the tiny atom are the powers to both create and destroy. Through fusion and fission man has learned to unleash the power of the atom. Our sun and other stars use fusion of the atom to create the heavier elements in the universe that were not created in the Big Bang. Fission of the atom is used to create power in nuclear power plants. Fusion of the atom may one day be used to create safer forms of power than current fuels that are destroying the delicate balance of earth's ecosystem.

External links

- [http://www.howstuffworks.com/atom.htm How Atoms Work] ko:원자 ms:Atom ja:原子 simple:Atom th:อะตอม

Molecules

A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. The science of molecules is called molecular chemistry or molecular physics, depending on the focus. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, however, this distinction is vague. According to the strict definition, molecules can consist of one atom (as in noble gases) or more atoms bonded together. The concept of monatomic (single-atom) molecule is used almost exclusively in the kinetic theory of gases. In molecular sciences, a molecule consists of a stable system (bound state) comprising two or more atoms. The term unstable molecule is used for very reactive species, i.e., short-lived assemblies (resonances) of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, Van der Waals complexes, or systems of colliding atoms as in Bose-Einstein condensates. A peculiar use of the term molecular is as a synonym to covalent, which arises from the fact that, unlike molecular covalent compounds, ionic compounds do not yield well-defined smallest particles that would be consistent with the definition above. No typical "smallest particle" can be defined for covalent crystals, or network solids, which are composed of repeating unit cells that extend indefinitely either in a plane (such as in graphite) or three-dimensionally (such as in diamond). Although the concept of molecules was first introduced in 1811 by Avogadro, and was accepted by many chemists as a result of Dalton's laws of Definite and Multiple Proportions (1803-1808), with notable exceptions (Boltzmann, Maxwell, Gibbs), the existence of molecules as anything other than convenient mathematical constructs was still an open debate in the physics community until the work of Perrin (1911), and was strenuously resisted by early positvists such as Mach. The modern theory of molecules makes great use of the many numerical techniques offered by computational chemistry. Dozens of molecules have now been identified in interstellar space by microwave spectroscopy. microwave spectroscopy (right) representations of the terpenoid, atisane. In the 3D model on the left, carbon atoms are represented by gray spheres; white spheres represent the hydrogen atoms and the cylinders represent the bonds. The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) electric charge. In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single bonds.]]

Chemical bond

:See main article chemical bond In a molecule, the atoms are joined by shared pairs of electrons in a chemical bond. It may consist of atoms of the same chemical element, as with oxygen (O₂), or of different elements, as with water (H₂O).

Size

Most molecules are much too small to be seen with the naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes. The smallest molecule is the hydrogen molecule. The interatomic distance is 0.15 nanometres (1.5 Å). But the size of its electron cloud is difficult to define precisely. Under standard conditions molecules have a dimension of a few to a few dozen Å.

Empirical formula

:See main article empirical formula The empirical formula of a molecule is the simplest integer ratio of the chemical elements that constitute the compound. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers. The empirical formula is often the same as the molecular formula but not always. For example the molecule acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH.

Chemical formula

:See main article chemical formula The chemical formula reflects the exact number of atoms that compose a molecule. The molecular mass can be calculated from the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a ¹²C isotope atom. For network solids, the term formula unit is used in stoichiometric calculations.

Molecular geometry

:See main article molecular geometry Molecules have fixed equilibrium geometries—bond lengths and angles—. A pure substance is composed of molecules with the same geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity. Isomers share a chemical formula but normally have very different properties because of their different structures. Stereoisomers, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different biochemical activities.

Molecular spectroscopy

:See main article spectroscopy Molecular spectroscopy is the study of the response (spectrum) of a molecule to a signal of known energy (or frequency, according to Planck's formula). This signal is usually an electromagnetic wave or a beam of electrons, but new molecular spectroscopies, such as the positron spectroscopy, are under development. The molecular response can be signal absorption (absorption spectroscopy), emission of another signal (emission spectroscopy), fragmentation, or a change in its chemical nature. Spectroscopy is recognized as the most powerful tool in the investigation of the microscopic properties of molecules, and, in particular, their energy levels. Nowadays, in order to extract the maximum microscopic information from the experimental results, spectroscopical studies are very often coupled with computational chemical investigations. The theoretical background of spectroscopy is the scattering theory.

Chemical reaction

A chemical reaction is a process that results in the interconversion of chemical substances . The substance(s) initially involved in a chemical reaction are called reactants. Chemical reactions are characterized by a chemical change and it yields one or more product(s) which are different from the reactants. Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds, although the general concept of a chemical reaction, in particular the notion of a chemical equation, is applicable to transformations of elementary particles, as well as nuclear reactions. Many different chemical reactions are used in combinations in chemical synthesis in order to get a desired product. In biochemistry, series of chemical reactions form metabolic pathways, since straight synthesis of a product would be energetically impossible in conditions within a cell. Chemical reactions are also divided into organic reactions and inorganic reactions.

Reaction types

There are five major classifications of chemical reactions. Some common and widely used terms are:
- Isomerization in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition; see stereoisomerism
- Direct combination or synthesis, in which two or more chemical element or compounds unite to form a more complex product; f.e. formation of water from hydrogen and oxygen
- Chemical decomposition or analysis, in which a compound is decomposed into smaller compounds; f.e. combustion of hydrocarbons
- Single displacement or substitution, characterized by an element being displaced out of a compound by a more reactive element; f.e. acid-base reactions
- Double displacement or coupling substitution , in which two compounds in aqueous solution (usually ionic) exchange elements or ions to form different compounds. Some branches of chemistry include any minor changes in chemical conformation in the reaction types, while others consider these changes merely as physical properties of a compound. The collision of more than two particles into the ordered structure necessary to perform chemical transformations is extremely unlikely; which is why ternary reactions in practice are not observed. A chemical reaction may require three or more reagents, but the process can generally be decomposed into a stepwise series or a set of stepwise reactions of the above. The large diversity of chemical reactions makes it difficult to establish simple criteria for functional (as opposed to mechanistic) classification. However, some kinds of reactions have similarities which make it possible to define some larger groups. A few examples are:
- Organic reactions, which encompass several different kinds of reactions involving compounds which have carbon as the main element in their molecular structure. These reactions occur mostly according to, within, by, or via functional groups. Reactions in petrochemistry aren't always classified as organic.
- Redox reactions, which involve augmenting or decreasing the electrons associated with a particular atom. according to its oxidation number.
- Combustion, where a substance reacts with oxygen gas;

Thermochemistry

See main article: Thermochemistry. Thermochemistry deciphers whether a specific chemical reaction can or cannot occur. Thermodynamics (or what is now known as equilibrium thermodynamics) understands the reaction in terms of the initial and final states of the reaction mixture. Reactions very seldom occur directly. Usually, reactants must collide to form an activated complex. This complex has a higher internal energy than the original reactants combined, having gained some from the kinetic energy of the reactant substances' collision. This energy allows for the rearrangement of bonds which constitutes the reaction. In some reactions, the reactants may pass through several reactive intermediates before becoming products. Thermodynamics does not attempt to figure out the process by which a reaction occurs. This field of study is taken up by the field of chemical kinetics. Another question "How fast is the reaction?" is also left completely unanswered by it. Chemical kinetics attempts to put all these phenomena into perspective.

Chemical equilibrium

Every chemical reaction is, in theory, reversible. In a forward reaction the substances defined as reactants are converted to products. In a reverse reaction products are converted into reactants. Chemical equilibrium is the state in which the forward and reverse reaction rates are equal, thus preserving the amount of reactants and products. However, a reaction in equilibrium can be driven in the forward or reverse direction. This is done by changing the reaction conditions such as temperature or pressure. Le Chatelier's principle can be used to predict whether products or reactants will be formed. Although all reactions are reversible to some extent, some reactions can be classified as irreversible. An irreversible reaction is one that "goes to completion." This phrase means that nearly all of the reactants are used to form products. These reactions are very difficult to reverse even under extreme conditions.

Exothermic reactions

Le Chatelier's principle According to energy balance criteria, that is, chemical reaction equilibria criteria, any closed system will tend to minimize its free energy. Without any outside influence, any reaction mixture, too, will try to do the same. For many cases, an analysis of the enthalpy of the system will give a decent account of the energetics of the reaction mixture. The enthalpy of a reaction is calculated using standard reaction enthalpies and the Hess' law of constant heat summation. Many of these enthalpies may be found in beginners' books on thermodynamics. For example, consider the combustion of methane in oxygen: :CH₄ + 2 O₂ → CO₂ + 2 H₂O By calculating the amounts of energy required to break all the bonds on the left ("before") and right ("after") sides of the equation using collected data, it is possible to calculate the energy difference between the reactants and the products. This is referred to as ΔH, where Δ (Delta) means difference, and H stands for enthalpy, a measure of energy which is equal to the heat transferred at constant pressure. ΔH is usually given in units of kilojoules (kJ) or in kilocalories (kcal). If ΔH is negative for the reaction, then energy has been released often in the form of heat. This type of reaction is referred to as an exothermic reaction (literally, outside heat, or throwing off heat). An exothermic reaction is more favourable and thus more likely to occur. An example reaction is combustion, known from everyday experience, since burning gas in air produces heat.

Endothermic reactions

combustion A reaction may have a positive ΔH. If a reaction has a positive ΔH, it consumes energy as the reaction moves towards completion. This type of reaction is called an endothermic reaction (literally, inside heat, or absorbing heat). The above rule, "Exothermic reactions are favourable", is usually true. However, there may be situations where exothermic reactions may not be favourable. This happens when the stability obtained due to loss of enthalpy is off set by a corresponding decrease in entropy (a measure of disorder). The exact rule is that a reaction is favourable when the Gibbs free energy of that reaction is negative where ΔG = ΔH − TΔS; ΔG being the change in Gibbs free energy, ΔH being the change in enthalpy, and ΔS is the change in entropy A reaction is called spontaneous if its thermodynamically favoured, by that meaning that it causes a net increase on entropy. Spontaneous reactions (in opposition to non-spontaneous reactions) do not need external perturbations (such as energy supplement) to happen. In a system at chemical equilibrium, it is expected to have larger concentrations of the substances formed by the spontaneous direction of the process. Thus, in a global isolated system (which it strictly isn't, see entropy), spontaneous reactions may be understood to occur without human interference. Most spontaneus reactions in this system are exothermic (such as rusting) or metamorphosis, thus increasing the global entropy, though photosynthesis is an important exeption (in a global system).

Chemical kinetics

See main article: Chemical kinetics. The rate of a chemical reaction is a measure of how the concentration of the involved substances changes with time. Analysis of reaction rates is important for several applications, such as in chemical engineering or in chemical equilibrium study. Rates of reaction depends basically on:
- Reactant concentrations, which usually make the reaction happen at a faster rate if raised,
- Surface Area, the amount of the substance being used,
- Pressure, By increasing the pressure, you squeeze the molecules together so you will increase the frequency of collisions between the molecules.
- Activation energy, which is defined as the amount of energy required to make the reaction start and carry on spontaneously. Higher activation energy implies that a reaction will be harder to start and, therefore, slower.
- Temperature, which hastens reactions if raised, because higher temperature means that the involved species will have more energy, thus making the reaction easier to happen,
- The presence or absence of a catalyst. Catalysts are substances which increases the speed of a reaction by lowering the activation energy needed for the reaction to take place. A catalyst is not destroyed or changed during a reaction, so it can be used again. Reaction rates are related to the concentrations of substances involved in reactions, as quantified by the law of mass action. Reactions whose rates are independent of reac

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