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[[Image:Chemicals in flasks.jpg|thumb|250px|right|Chemistry is the [[science]] concerned with  the composition, structure, and properties of [[matter]], as well as the changes it undergoes during [[chemical reaction]]s.]]
[[Image:VysokePece1.jpg|thumb|250px|right|'''Chemistry''' - is the study of interactions of chemical substances with one another and energy ]] 
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==Overview==
'''Chemistry''' (from [[Egyptian language|Egyptian]] ''kēme'' (chem), meaning [[Classical element|"earth"]]<ref>'''See:''' [[Chemistry (etymology)]] for possible origins of this word.</ref>) is the [[science]] concerned with  the composition, structure, and properties of [[matter]], as well as the changes it undergoes during [[chemical reaction]]s.<ref>[http://dictionary.reference.com/browse/Chemistry Chemistry]. (n.d.). Merriam-Webster's Medical Dictionary. Retrieved [[August 19]], [[2007]].</ref> Chemistry is a [[physical science]] related to studies of various [[atom]]s, [[molecule]]s, [[crystal]]s and other  aggregrates of matter whether in isolation or combination, which incorporates the concepts of [[energy]] and [[entropy]] in relation to the [[spontaneous reaction|spontaneity]] of [[chemical process]]es.

Disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include [[inorganic chemistry]], the study of [[inorganic]] matter; [[organic chemistry]], the study of [[Organic compound|organic]] matter; [[biochemistry]], the study of [[chemical substance|substances]] found in [[organisms|biological organisms]]; [[physical chemistry]], the [[energy]] related studies of chemical systems at macro, molecular and submolecular scales; [[analytical chemistry]], the analysis of material samples to gain an understanding of their [[chemical composition]] and [[Chemical structure|structure]]. Many more specialized disciplines have emerged in recent years, e.g. [[neurochemistry]] the chemical study of the mind (see [[#Subdisciplines|subdisciplines]]).  

Historically, modern chemistry evolved out of [[alchemy]] following the [[chemical revolution]] (1773) (see [[#History|History]]).

==Overview==

Chemistry is the scientific study of interaction of substances called [[chemical substance]]s<ref>[http://chemweb.ucc.ie/what_is_chemistry.htm What is Chemistry?]</ref> that are constituted of [[atom]]s or the subatomic components that make up atoms: [[proton]]s, [[electron]]s and [[neutron]]s.<ref>[http://www.visionlearning.com/library/module_viewer.php?mid=49 Matter: Atoms from Democritus to Dalton] by Anthony Carpi, Ph.D.</ref> Atoms combine to produce [[ion]]s, [[molecule]]s or [[crystal]]s. Chemistry can be called "[[the central science]]" because it connects the other [[natural science]]s, such as [[astronomy]], [[physics]], [[material science]], [[biology]], and [[geology]].<ref>Theodore L. Brown, H. Eugene Lemay, Bruce Edward Bursten, H. Lemay. ''Chemistry: The Central Science''. Prentice Hall; 8 edition (1999). ISBN 0130103101. Pages 3-4.</ref><ref>It is sometimes called the central science because it is seen as occupying an intermediate position in a hierarchy of the sciences by "reductive level", between physics and biology. See Carsten Reinhardt. ''Chemical Sciences in the 20th Century: Bridging Boundaries''. Wiley-VCH, 2001. ISBN 3527302719. Pages 1-2.</ref> 

The genesis of chemistry can be traced to certain practices, known as [[alchemy]], which had been practiced for several [[millennia]] in various parts of the world, particularly the middle east.<ref>Dictionary of the History of Ideas: Alchemy [http://etext.lib.virginia.edu/cgi-local/DHI/dhi.cgi?id=dv1-04]</ref>

The structure of objects we commonly use and the properties of the matter we commonly interact with, are a consequence of the properties of chemical substances and their interactions. For example, [[steel]] is [[Hardness (materials science)|harder]] than iron because its atoms are bound together in a more rigid [[crystalline lattice]]; wood burns or undergoes rapid [[oxidation]] because it can react spontaneously with [[oxygen]] in a [[chemical reaction]] above a certain [[temperature]]; sugar and salt dissolve in water because their molecular/ionic properties are such that dissolution is preferred under the ambient conditions. 

The transformations that are studied in chemistry are a result of interaction either between different chemical substances or between [[matter]] and [[energy]]. Traditional chemistry involves study of [[interactions]] between [[chemical substance|substances]] in a chemistry [[laboratory]] using various forms of [[laboratory glassware]]. [[Image:Lab bench.jpg|thumb|300px|left|Laboratory, Institute of Biochemistry, University of Cologne]] A [[chemical reaction]] is a transformation of some such substances into one or more other such substances.<ref>IUPAC [[Gold Book]] [http://www.iupac.org/goldbook/C01033.pdf Definition]</ref>  It can be symbolically depicted through a [[chemical equation]].  The number of atoms on the left and the right in the equation for a chemical transformation is most often equal. The nature of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws.

[[Energy]] and [[entropy]] considerations are invariably important in almost all chemical studies. Chemical substances are classified in terms of their [[structure]], phase as well as their [[chemical composition]]s. They can be analysed using the tools of [[chemical analysis]], e.g. [[spectroscopy]] and [[chromatography]]. 

Chemistry is an integral part of the [[science]] curriculum both at the [[high school]] as well as the early college level. At these levels, it is often called 'general chemistry' which is an introduction to a wide variety of fundamental concepts that enable the student to acquire tools and skills useful at the advanced levels, whereby chemistry is invariably studied in any of its various sub-disciplines. [[Scientists]], engaged in  chemical [[research]] are known as [[chemists]].<ref>California Occupational Guide Number 22: Chemists[http://www.calmis.ca.gov/file/occguide/CHEMIST.HTM]</ref> Most chemists specialize in one or more sub-disciplines.

==History==

{{main|History of chemistry}}
{{see also|Alchemy|Timeline of chemistry|Nobel Prize in Chemistry}}

The genesis of chemistry can be traced to the widely observed phenomenon of [[combustion|burning]] that led to [[metallurgy]]- the art and science of processing ores to get metals (e.g. [[History of metallurgy in the Indian subcontinent|metallurgy in ancient India]]). The greed for gold led to the discovery of the process for its purification, even though, the underlying principles were not well understood -- it was thought to be a transformation rather than purification. Many scholars in those days thought it reasonable to believe that there exist means for transforming cheaper (base) metals into gold. This gave way to alchemy, and the search for the [[Philosopher's Stone]], which was believed to bring about such a transformation by mere touch.<ref>[http://www.chemheritage.org/explore/ancients-time.html Alchemy Timeline] - Chemical Heritage Society</ref>

Some consider [[Alchemy (Islam)|medieval Muslims]] to be the earliest chemists, who introduced precise [[observation]] and controlled [[experiment]]ation into the field, and discovered numerous [[chemical substance]]s.<ref>[[Will Durant]] (1980), ''The Age of Faith (The Story of Civilization, Volume 4)'', p. 162-186, Simon & Schuster, ISBN 0671012002:
<br>{{quote|"Chemistry as a science was almost created by the Muslims; for in this field, where the Greeks (so far as we know) were confined to industrial experience and vague [[hypothesis]], the [[Saracen]]s introduced precise [[observation]], controlled [[experiment]], and careful records. They invented and named the [[alembic]] (al-anbiq), chemically analyzed innumerable [[Chemical substance|substance]]s, composed [[Lapidary|lapidaries]], distinguished [[alkali]]s and [[acid]]s, investigated their affinities, studied and manufactured hundreds of [[drug]]s. Alchemy, which the Muslims inherited from Egypt, contributed to chemistry by a thousand incidental discoveries, and by its method, which was the most scientific of all medieval operations."}}</ref> The most influential Muslim chemists were [[Geber]] (d. 815), [[al-Kindi]] (d. 873), [[al-Razi]] (d. 925), and [[Abū Rayhān al-Bīrūnī|al-Biruni]] (d. 1048).<ref>Dr. K. Ajram (1992), ''Miracle of Islamic Science'', Appendix B, Knowledge House Publishers, ISBN 0911119434.<br>{{quote|"[[Alexander von Humboldt|Humboldt]] regards the Muslims as the founders of chemistry."}}</ref> The works of Geber became more widely known in Europe through [[Latin]] translations by a [[pseudo-Geber]] in 14th century [[Spain]], who also wrote some of his own books under the pen name "Geber". The contribution of [[History of metallurgy in the Indian subcontinent|Indian alchemists and metallurgists]] in the development of chemistry was also quite significant.<ref>[[Will Durant]] (1935): Our Oriental Heritage: Simon & Schuster:
<br>{{quote|"Something has been said about the chemical excellence of cast iron in ancient India, and about the high industrial development of the Gupta times, when India was looked to, even by Imperial Rome, as the most skilled of the nations in such chemical industries as dyeing, tanning, soap-making, glass and cement... By the sixth century the Hindus were far ahead of Europe in industrial chemistry; they were masters of calcinations, distillation, sublimation, steaming, fixation, the production of light without heat, the mixing of anesthetic and soporific powders, and the preparation of metallic salts, compounds and alloys. The tempering of steel was brought in ancient India to a perfection unknown in Europe till our own times; King Porus is said to have selected, as a specially valuable gift from Alexander, not gold or silver, but thirty pounds of steel. The Moslems took much of this Hindu chemical science and industry to the Near East and Europe; the secret of manufacturing "Damascus" blades, for example, was taken by the Arabs from the Persians, and by the Persians from India.""}}</ref> 

The emergence of chemistry in Europe was primarily due to the recurrent incidence of the [[Bubonic plague|plague]] and blights there during the so called [[Dark Ages]]. This gave rise to a need for medicines. It was thought that there exists a universal medicine called the [[Elixir of Life]] that can cure all diseases, but like the Philosopher's Stone, it was never found. 

For some practitioners, alchemy was an intellectual pursuit, over time, they got better at it. [[Paracelsus]] (1493-1541), for example, rejected the 4-elemental theory and with only a vague understanding of his chemicals and medicines, formed a hybrid of alchemy and science in what was to be called  ''[[iatrochemistry]]''. Similarly, the influences of philosophers such as [[Sir Francis Bacon]] (1561-1626) and [[René Descartes]] (1596-1650), who demanded more rigor in mathematics and in removing bias from scientific observations, led to a [[scientific revolution]]. In chemistry, this began with [[Robert Boyle]] (1627-1691), who came up with an equations known as the [[Boyle's Law]] about the characteristics of gaseous state.<ref> [http://www.bbc.co.uk/history/historic_figures/boyle_robert.shtml BBC - History - Robert Boyle (1627 - 1691)]</ref> Chemistry indeed came of age when [[Antoine Lavoisier]] (1743-1794), developed the theory of [[Conservation of mass]] in 1783; and the development of the [[Atomic Theory]] by [[John Dalton]] around 1800.  The Law of Conservation of Mass resulted in the reformulation of chemistry based on this law and the oxygen theory of combustion, which was largely based on the work of Lavoisier.  Lavoisier's fundamental contributions to chemistry were a result of a conscious effort to fit all experiments into the framework of a single theory. He established the consistent use of the chemical balance, used oxygen to overthrow the [[phlogiston theory]], and developed a new system of chemical nomenclature and made contibution to the modern metric system.  Lavoisier  also worked to translate the archaic and technical language of chemistry into something that could be easily understood by the largely uneducated masses, leading to an increased public interest in chemistry. All these advances in chemistry led to what is usually called the [[chemical revolution]].  The contributions of Lavoisier led to what is now called modern chemistry - the chemistry that is studied in educational institutions all over the world.  It is because of these and other contribtuions that Antoine Lavoisier is often celebrated as the "Father of Modern Chemistry".  The later discovery of [[Friedrich Wöhler]] that many natural substances, [[organic compound]]s, can indeed be synthesized in a chemistry [[laboratory]] also helped the modern chemistry to mature from its infancy.

The [[discoveries of the chemical elements]] has a long history from the days of alchemy and culminating in the creation of the [[periodic table]] of the chemical elements by [[Dmitri Mendeleev]] (1834-1907)<ref> [http://chemistry.about.com/library/weekly/aa030303a.htm Timeline of Element Discovery] - About.com</ref> and later discoveries of some [[synthetic elements]].

==Etymology==
{{main|Chemistry (etymology)}}
The word ''chemistry'' comes from the earlier study of alchemy, which is basically the quest to make gold from earthen starting materials.<ref>Alchemy Lab: History of Alchemy [http://www.alchemylab.com/history_of_alchemy.htm]</ref> As to the origin of the word "alchemy" the question is a debatable one; it certainly can be traced back to the Greeks, and some, following E. Wallis Budge, have also asserted [[Ancient Egypt|Egyptian]] origins. Alchemy, generally, derives from the old French ''alkemie'' from the Arabic ''al-kimia'' - "the art of transformation". The Arabs borrowed the word "kimia" from the Greeks when they conquered [[Alexandria]] in the year 642 AD. A tentative outline is as follows:

#Egyptian alchemy [5,000 BCE &ndash; 400 BCE], formulate early "element" theories such as the [[Ogdoad]].
#Greek alchemy [332 BCE &ndash; 642 CE], the Greek king [[Alexander the Great]] conquers Egypt and founds Alexandria, having the world's largest library, where scholars and wise men gather to study.
#[[Alchemy (Islam)|Arabian alchemy]] [642 CE &ndash; 1200], the Arabs take over Alexandria; [[Geber|Jabir]] is the main chemist
#European alchemy [1300 &ndash; present], [[Pseudo-Geber]] builds on Arabic chemistry
#Chemistry [1661], [[Robert Boyle|Boyle]] writes his classic chemistry text ''The Sceptical Chymist''
#Chemistry [1787], [[Antoine Lavoisier|Lavoisier]] writes his classic ''Elements of Chemistry''
#Chemistry [1803], [[John Dalton|Dalton]] publishes his ''Atomic Theory''

Thus, an alchemist was called a 'chemist' in popular speech, and later the suffix "-ry" was added to this to describe the art of the chemist as "chemistry".

==Definitions==
In retrospect, the definition of chemistry seems to invariably change per decade, as new discoveries and theories add to the functionality of the science. Shown below are some of the standard definitions used by various noted chemists:

*'''Alchemy''' (330) – the study of the composition of waters, movement, growth, embodying and disembodying, drawing the spirits from bodies and bonding the spirits within bodies ([[Zosimos of Panopolis|Zosimos]]).<ref>Strathern, P. (2000). ''Mendeleyev’s Dream – the Quest for the Elements.'' New York: Berkley Books.</ref>
*'''Chymistry''' (1661) – the subject of the material principles of mixt bodies ([[Robert Boyle|Boyle]]).<ref>{{cite book| last=Boyle | first = Robert |title=The Sceptical Chymist|location=New York | publisher=Dover Publications, Inc. (reprint)|year=1661|id=ISBN 0486428257}}</ref>
*'''Chymistry''' (1663) – a scientific art, by which one learns to dissolve bodies, and draw from them the different substances on their composition, and how to unite them again, and exalt them to an higher perfection ([[Christopher Glaser|Glaser]]).<ref>{{cite book| last=Glaser | first = Christopher |title=Traite de la chymie|location=Paris | year=1663}} as found in: {{cite book | last = Kim | first = Mi Gyung | title = Affinity, That Elusive Dream - A Genealogy of the Chemical Revolution | publisher = The MIT Press | year = 2003 | id = ISBN 0-262-11273-6}}
</ref>
*'''Chemistry''' (1730) – the art of resolving mixt, compound, or aggregate bodies into their principles; and of composing such bodies from those principles ([[Georg Ernst Stahl|Stahl]]).<ref>{{cite book| last=Stahl | first = George, E. |title=Philosophical Principles of Universal Chemistry|location=London | year=1730}}</ref>
*'''Chemistry''' (1837) – the science concerned with the laws and effects of molecular forces ([[Jean-Baptiste Dumas|Dumas]]).<ref>Dumas, J. B. (1837). 'Affinite' (lecture notes), vii, pg 4. “Statique chimique”, Paris: Academie des Sciences</ref>
*'''Chemistry''' (1947) – the science of substances: their structure, their properties, and the reactions that change them into other substances ([[Linus Pauling|Pauling]]).<ref>{{cite book | last = Pauling | first = Linus  | title = General Chemistry | publisher = Dover Publications, Inc. | year = 1947 | id = ISBN 0486656225}}</ref>
*'''Chemistry''' (1998) – the study of matter and the changes it undergoes ([[Raymond Chang|Chang]]).<ref>{{cite book|author=Chang, Raymond |title=Chemistry, 6th Ed.|location=New York | publisher=McGraw Hill|year=1998|id=ISBN 0-07-115221-0}}</ref>

==Basic concepts==

Several [[concepts]] are essential for the study of chemistry, some of them are:<ref>General Chemistry Online - Companion Notes: Matter [http://antoine.frostburg.edu/chem/senese/101/matter/]</ref> 

===Atom===
{{main|Atom}}

An ''atom'' is the basic unit of an element. It is a collection of matter consisting of a positively charged core (the [[atomic nucleus]]) which contains [[protons]] and [[neutrons]], and which maintains a number of [[electron]]s to balance the positive charge in the nucleus. Atom is also the smallest entity that can be envisaged to retain some of the [[chemical properties]] of the element, viz. [[Electronegativity]], [[Ionization potential]], Preferred [[oxidation state]](s), [[Coordination number]], Preferred types of [[chemical bond|bonds]] to form e.g., [[metallic]], [[ion]]ic, [[covalent]].

===Element===
{{main|Chemical element}}

The concept of ''chemical element'' is related to that of chemical substance.  A chemical element is characterized by a particular number of [[proton]]s in the [[atomic nucleus|nuclei]] of its atoms. 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]]. However, several [[isotope]]s of an element, that differ from one another in the number of neutrons present in the nucleus, may exist.

The most convenient presentation of the chemical elements is in the [[periodic table]] of the chemical elements, which groups elements by atomic number. Due to its ingenious arrangement, [[Periodic table group|groups]], or columns, and [[periods]], or rows, of elements in the table either share several chemical properties, or follow a certain trend in characteristics such as [[atomic radius]], [[electronegativity]], etc. Lists of the elements [[List of elements by name|by name]], [[List of elements by symbol|by symbol]], and by [[List of elements by number|atomic number]] are also available.

===Compound===
{{main|Chemical compound}}

A ''compound'' is a substance with a ''particular ratio'' of atoms of particular [[chemical element]]s which determines its composition, and a particular [[chemical structure|organization]] which determines chemical properties. For example, [[water (molecule)|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 reaction]]s.

=== Substance ===
{{main|Chemical substance}}

A chemical substance is a kind of matter with a definite [[chemical composition|composition]] and set of [[chemical properties|properties]]. Strictly speaking, a mixture of compounds, elements or compounds and elements is not a chemical substance, but it may be called a chemical. Most of the substances we encounter in our daily life are some kind of mixture, e.g. [[Earth's atmosphere|air]], [[alloy]]s, [[biomass]] etc.

Nomenclature of substances is a critical part of the language of chemistry. Generally it refers to a system for naming [[chemical compound]]s. Earlier in the history of chemistry substances were given name by their discoverer, which often led to some confusion and difficulty. However, today the IUPAC system of chemical nomenclature  allows chemists to specify by name specific compounds amongst the infinite variety of possible chemicals. The standard nomenclature of chemical substances is set by the [[International Union of Pure and Applied Chemistry]] (IUPAC). There are well-defined systems in place for naming chemical species.  [[Organic compound]]s are named according to the [[organic nomenclature]] system.<ref>IUPAC Nomenclature of Organic Chemistry [http://www.acdlabs.com/iupac/nomenclature/]</ref> [[Inorganic compound]]s are named according to the [[inorganic nomenclature]] system.<ref>IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (2004) [http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html]</ref> In addition the [[Chemical Abstracts Service]] has devised a method to index chemical substance. In this scheme each chemical substance is identifiable by a numeric number known as [[CAS registry number]].

===Molecule===
{{main|Molecule}}

A ''molecule'' is the smallest indivisible portion, beside an atom, of a pure [[chemical substance]] that has its unique set of chemical properties, that is, its potential to undergo a certain [[set]] of chemical reactions with other substances. Molecules can exist as  electrically neutral units unlike [[ions]]. Molecules are typically a set of atoms bound together by [[covalent bond]]s, such that the structure is electrically neutral and all valence electrons are paired with other electrons either in bonds or in [[lone pair]]s.
[[Image:TaxolTotalSynthesisColor.png|400px|right|thumb|A molecular structure depicts the bonds and relative positions of atoms in a molecule such as that in [[Paclitaxel]] shown here]]
One of the main characteristic of a molecule is its geometry often called its [[molecular structure|structure]]. While the structure of diatomic, triatomic or tetra atomic molecules may be trivial, (linear, angular pyramidal etc.) the structure of polyatomic molecules, that are constituted of more than six atoms (of several elements) can be crucial for its chemical nature.

===Mole===

{{Main|Mole (unit)}}
A mole is the amount of a  [[Chemical substance|substance]] that contains as many elementary entities (atoms, molecules or ions) as there are atoms in 0.012 [[kilogram]] (or 12 [[gram]]s) of [[carbon-12]], where the carbon-12 atoms are unbound, at rest and in their [[ground state]].<ref>[http://www.bipm.org/en/si/base_units/ Official SI Unit definitions]</ref> This number is known as the [[Avogadro constant]], and is determined empirically. The currently accepted value is 6.02214179(30){{e|23}} mol<sup>-1</sup> (2007 [[CODATA]]). It is much like the term "a [[dozen]]" in that it is an absolute number (having no units) and can describe any type of elementary object, although the mole's use is usually limited to measurement of [[subatomic]], [[atom]]ic, and [[molecule|molecular]] structures.

The number of moles of a substance in one liter of a [[solution]] is known as its [[molarity]]. Molarity is the common unit used to express the [[concentration]] of a solution in [[physical chemistry]].

=== Ions and salts===
{{main|Ion}}

An ''ion'' is a charged species, an atom or a molecule, that has lost or gained one or more electrons. Positively charged [[cations]] (e.g. [[sodium]] cation Na<sup>+</sup>) and negatively charged [[anions]] (e.g. [[chloride]] Cl<sup>−</sup>) can form a crystalline lattice of neutral [[salt]]s (e.g. [[sodium chloride]] NaCl). Examples of [[polyatomic ion]]s that do not split up during [[Acid-base reaction theories|acid-base reactions]] are [[hydroxide]] (OH<sup>−</sup>) and [[phosphate]] (PO<sub>4</sub><sup>3−</sup>).

Ions in the gaseous phase is often known as [[plasma (physics)|plasma]].

===Phase===
{{main|Phase (matter)}}

In addition to the specific chemical properties that distinguish different chemical classifications chemicals can exist in several phases. For the most part, the chemical classifications are independent of these bulk phase classifications; however, some more exotic phases are incompatible with certain chemical properties. 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 fluid|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 [[solid]]s, [[liquid]]s, and [[gas]]es. Many substances exhibit multiple solid phases.  For example, there are three phases of solid [[iron]] (alpha, gamma, and delta) that vary based on temperature and pressure.  A principal difference between solid phases is the [[crystal structure]], or arrangement, of the atoms.  Less familiar phases include [[Plasma physics|plasmas]], [[Bose-Einstein condensate]]s and [[fermionic condensate]]s and the [[paramagnetism|paramagnetic]] and [[ferromagnetism|ferromagnetic]] phases of [[magnet]]ic materials. While most familiar phases deal with three-dimensional systems, it is also possible to define analogs in two-dimensional systems, which has received attention for its relevance to systems in [[biology]].

===Chemical bond===
{{main|Chemical bond}}
[[Image:Electron orbitals.svg|right|thumb|350px|[[Electron]] atomic and [[molecular orbital|molecular]] orbitals]]

A ''chemical bond'' is a concept for understanding how atoms stick together in molecules. It may be visualized as the [[multipole]] balance between the positive charges in the nuclei and the negative charges oscillating about them.<ref>visionlearning: Chemical Bonding by Anthony Carpi, Ph. [http://www.visionlearning.com/library/module_viewer.php?mid=55]</ref> More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom. These potentials create the [[interaction]]s which holds together [[atom]]s in [[molecule]]s or [[crystal]]s. In many simple compounds, [[Valence Bond Theory]], the Valence Shell Electron Pair Repulsion model ([[VSEPR]]), 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 [[complex (chemistry)|metal complexes]], valence bond theory fails and alternative approaches, primarily based on principles of [[quantum chemistry]] such as the [[molecular orbital]] theory, are necessary. See diagram on electronic orbitals.

===Chemical reaction===
{{main|Chemical reaction}}

''Chemical reaction'' is a concept related to the transformation of a [[chemical substance]] through its interaction with another, or  as a result of its interaction with some [[energy forms|form of energy]]. A chemical reaction may occur naturally or carried out in a laboratory by chemists in specially designed vessels which are often [[laboratory glassware]]. It can result in the formation or [[dissociation (chemistry)|dissociation]] of molecules, that is,  molecules breaking apart to form two or more smaller molecules, or rearrangement of [[atom]]s within or across molecules. Chemical reactions usually involve the making or breaking of [[chemical bond]]s. [[Redox|Oxidation, reduction]], [[dissociation (chemistry)|dissociation]], acid-base [[neutralization]] and molecular [[rearrangement]] are some of the commonly used kinds of chemical reactions.

A chemical reaction can be symbolically depicted through a [[chemical equation]]. While in a non-nuclear chemical reaction the number and kind of atoms on both sides of the equation are equal, for a nuclear reaction this holds true only for the nuclear particles viz. protons and neutrons.<ref>[http://goldbook.iupac.org/C01034.html Chemical Reaction Equation]- IUPAC Goldbook</ref> 

The sequence of steps in which the reorganization of chemical bonds may be taking place in the course of a chemical reaction is called its [[Reaction mechanism|mechanism]]. A chemical reaction can be envisioned to take place in a number of steps, each of which may have a different speed. Many [[reaction intermediates]] with variable stability can thus be envisaged during the course of a reaction. Reaction mechanisms are proposed to explain the [[chemical kinetics|kinetics]] and the relative product mix of a reaction. Many [[chemists|physical chemists]] specialize in exploring and proposing the mechanisms of various chemical reactions. Several empirical rules, like the [[Woodward-Hoffmann rules]] often come handy while proposing a mechanism for a chemical reaction.

A stricter definition is that "a chemical reaction is a process that results in the interconversion of chemical species".<ref>[[Gold Book]] [http://goldbook.iupac.org/C01033.html Chemical Reaction] IUPAC Goldbook</ref> Under this definition, a chemical reaction may be an [[elementary reaction]] or a [[stepwise reaction]]. An additional caveat is made, in that this definition includes cases where the [[conformer|interconversion of conformers]] is experimentally observable. Such detectable chemical reactions normally involve sets of molecular entities as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events').

===Energy===
{{Main|Energy}}
A [[chemical reaction]] is invariably accompanied by an [[endothermic reaction|increase]] or [[exothermic reaction|decrease]] of [[energy]] of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of [[heat]] or [[photochemistry|light]], thus the products of a reaction may have more or less energy than the reactants. A reaction is said to be [[exothermic]] if the final state is lower on the energy scale than the initial state; in case of [[endothermic]] reactions the situation is otherwise.

Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the [[activation energy]]. The ''speed'' of a  chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor <math>e^{-E/kT} </math> - that is the probability of molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the [[Arrhenius equation]].
The activation energy necessary for a chemical reaction can be in the form of heat, light, [[electricity]] or mechanical [[force]] in the form of [[ultrasound]].<ref>Reilly, Michael. (2007).
"[http://www.newscientisttech.com/article/dn11427 Mechanical force induces chemical reaction]", NewScientist.com news service, Reilly </ref>  

A related concept [[thermodynamic free energy|Free energy]], which incorporates entropy considerations too, is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, in [[chemical thermodynamics]]. A reaction is feasible only if the total change in the [[Gibbs free energy]] is negative, <math> \Delta G \le 0 \,</math>; if it is equal to zero the chemical reaction is said to be at [[chemical equilibrium|equilibrium]].

There are only a limited possible states of energy for electrons, atoms and molecules. These are determined by the rules of [[quantum mechanics]], which require [[quantization (physics)|quantization]] of energy of a bound system. The atoms/molecules in an higher energy state are said to be excited. The molecules/atoms of substance in an excited energy state are often much more reactive, that is amenable to chemical reactions.  

The phase of a substance is invariably determined by its energy and those of its surroundings. When the intermolecular forces of a substance are such that energy of the surroundings is not sufficient to overcome them, it occurs in a more ordered phase like liquid or solid as is the case with  water (H<sub>2</sub>O), a liquid at room temperature because its molecules are bound by [[hydrogen bonds]].<ref>[http://www.chem4kids.com/files/matter_changes.html Changing States of Matter] - Chemforkids.com</ref> Whereas [[hydrogen sulfide]] (H<sub>2</sub>S) is a gas at room temperature and standard pressure, as its molecules are bound by weaker [[Intermolecular force#Dipole-dipole interactions|dipole-dipole interactions]]. 

The transfer of energy from one chemical substance to other depend on the ''size'' of energy [[quanta]] emitted from one substance. However, heat energy is easily transferred from almost any substance to another mainly because the vibrational and rotational energy levels in a substance are very closely placed. Because, the electronic energy levels are not so closely spaced, ultraviolet electromagnetic radiation is not transferred with equal felicity, as is also the case with electrical energy.

The existence of characteristic energy levels for different [[chemical substance]]s is useful for their identification by the analysis of [[spectral lines]] of different kinds of spectra often used in chemical spectroscopy e.g. [[infrared spectroscopy|IR]], [[microwave spectroscopy|microwave]], [[NMR]], [[electron spin resonance|ESR]] etc. This is used to identify the composition of remote objects - like stars and far galaxies - by analyzing their radiation (see [[spectroscopy]]).  

[[Image:Emission spectrum-Fe.png|none|frame|Emission spectrum of [[iron]]]]

The term [[Energy#chemical energy|chemical energy]] is often used to indicate the potential of a chemical substance to undergo a transformation through a [[chemical reaction]] or transform other chemical substances.

===Chemical laws===
{{main|Chemical law}}

Chemical reactions are governed by certain laws, which have become fundamental concepts in chemistry. Some of them are:

* [[Law of conservation of mass]], according to the modern physics it is actually [[energy]] that is conserved, and that energy and mass are [[Mass-energy equivalence|related]]; a concept which becomes important in [[nuclear chemistry]]. 
* [[Conservation of energy|Law of conservation of Energy]] leads to the important concepts of [[Chemical equilibrium|equilibrium]], [[thermodynamics]], and [[chemical kinetics|kinetics]].
* [[Law of definite composition]], although in many systems (notably biomacromolecules and minerals) the ratios tend to require large numbers, and are frequently represented as a fraction.  
* [[Law of multiple proportions]]
* [[Hess's Law]]
* [[Beer-Lambert law]]
* [[Fick's law of diffusion]]
* [[Raoult's Law]]
* [[Henry's law]]
* [[Boyle's law]] (1662, relating pressure and volume)
* [[Charles's law]] (1787, relating volume and temperature) 
* [[Gay-Lussac's law]] (1809, relating pressure and temperature)
* [[Avogadro's law]]

==Subdisciplines==

Chemistry is typically divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry.<ref>The Canadian Encyclopedia: Chemistry Subdisciplines [http://www.thecanadianencyclopedia.com/index.cfm?PgNm=TCE&Params=A1ARTA0001555]</ref>

* [[Analytical chemistry]] is the analysis of material samples to gain an understanding of their [[chemical composition]] and [[Chemical structure|structure]]. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdisciplines of chemistry, excluding purely theoretical chemistry.

* [[Biochemistry]] is the study of the [[chemical compound|chemicals]], [[chemical reaction]]s and chemical [[interaction]]s that take place in living [[organism]]s. Biochemistry and organic chemistry are closely related, as in [[medicinal chemistry]] or [[neurochemistry]]. Biochemistry is also associated with [[molecular biology]] and [[genetics]].

* [[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]].

* [[Materials Science|Materials chemistry]] is the preparation, characterization, and understanding of substances with a useful function.  The field is a new breadth of study in graduate programs, and it integrates elements from all classical areas of chemistry with a focus on fundamental issues that are unique to materials.  Primary systems of study include the chemistry of condensed phases (solids, liquids, [[polymers]]) and [[interfaces]] between different phases.

* [[Nuclear chemistry]] is the study of how subatomic particles come together and make nuclei. Modern [[Nuclear transmutation|Transmutation]] is a large component of nuclear chemistry, and the [[table of nuclides]] is an important result and tool for this field.

* [[Organic chemistry]] is the study of the structure, properties, composition, mechanisms, and [[chemical reaction|reactions]] of [[organic compound]]s. An organic compound is defined as any compound based on a carbon skeleton.

* [[Physical chemistry]] is the study of the physical and fundamental 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]]. Physical chemistry involves the use of [[calculus]] in deriving equations. It is usually associated with [[quantum chemistry]] and theoretical chemistry. Physical chemistry is a distinct discipline from [[chemical physics]].

* [[Theoretical chemistry]] is the study of chemistry via fundamental 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 [[World War II|Second World War]], the development of computers has allowed a systematic development of [[computational chemistry]], which is the art of developing and applying [[computer program]]s for solving chemical problems. Theoretical chemistry has large overlap with (theoretical and experimental) [[condensed matter physics]] and [[molecular physics]].

Other fields include [[Astrochemistry]], [[Atmospheric chemistry]], [[Chemical Engineering]], [[Chemical biology]], [[Chemo-informatics]], [[Electrochemistry]], [[Environmental chemistry]], [[Flow chemistry]], [[Geochemistry]], [[Green chemistry]], [[History of chemistry]], [[Materials science]], [[Medicinal chemistry]], [[Molecular Biology]], [[Nanotechnology]], [[Organometallic chemistry]], [[Petrochemistry]], [[Pharmacology]], [[Photochemistry]], [[Phytochemistry]], [[Polymer chemistry]], [[Solid-state chemistry]], [[Sonochemistry]], [[Supramolecular chemistry]], [[Surface chemistry]], [[Immunochemistry]] and [[Thermochemistry]].

==Chemical industry==
{{main|chemical industry}}

The [[chemical industry]] represents an important economic activity. The global top 50 chemical producers in 2004 had sales of 587 billion [[US dollars]] with a profit margin of 8.1% and [[research and development]] spending of 2.1% of total chemical sales.<ref>{{cite journal | title = Top 50 Chemical Producers | journal = Chemical & Engineering News | date = [[July 18]], [[2005]] | volume = 83 | issue = 29 | pages = 20–23 | url = http://pubs.acs.org/cen/coverstory/83/8329globaltop50.html}}</ref>

==Further reading==
===Popular reading===
*Atkins, P.W. ''Galileo's Finger'' (Oxford University Press) ISBN 0198609418
*Atkins, P.W. ''Atkins' Molecules'' (Cambridge University Press) ISBN 0521823978
*Stwertka, A. ''A Guide to the Elements'' (Oxford University Press) ISBN 0195150279

===Introductory undergraduate text books===
*Chang, Raymond. ''Chemistry'' 6th ed. Boston: James M. Smith, 1998. ISBN 0-07-115221-0.
*Atkins, P.W., Overton, T., Rourke, J., Weller, M. and Armstrong, F. ''Shriver and Atkins inorganic chemistry'' (4th edition) 2006 (Oxford University Press) ISBN 0-19-926463-5
*Clayden, J., Greeves, N., Warren, S., Wothers, P. ''Organic Chemistry'' 2000 (Oxford University Press) ISBN 0-19-850346-6
*Voet and Voet ''Biochemistry'' (Wiley) ISBN 0-471-58651-X

===Advanced undergraduate-level or graduate text books===
*Atkins, P.W. ''Physical Chemistry'' (Oxford University Press) ISBN 0-19-879285-9
*Atkins, P.W. et al. ''Molecular Quantum Mechanics'' (Oxford University Press)
*McWeeny, R. ''Coulson's Valence'' (Oxford Science Publications) ISBN 0-19-855144-4
*Pauling, L. ''The Nature of the chemical bond'' (Cornell University Press) ISBN 0-8014-0333-2
*Pauling, L., and Wilson, E. B. ''Introduction to Quantum Mechanics with Applications to Chemistry'' (Dover Publications) ISBN 0-486-64871-0
*Stephenson, G. ''Mathematical Methods for Science Students'' (Longman)ISBN 0-582-44416-0
*Smart and Moore ''Solid State Chemistry: An Introduction'' (Chapman and Hall) ISBN 0-412-40040-5

==Professional societies==
*[[American Chemical Society]]
*[[Chemical Institute of Canada]]
*[[Chemical Society of Peru]]
*[[International Union of Pure and Applied Chemistry]]
*[[Royal Australian Chemical Institute]]
*[[Royal Society of Chemistry]]
*[[Society of Chemical Industry]]
*[[World Association of Theoretical and Computational Chemists]]

==See also==
===Lists===
<div style="-moz-column-count:2; column-count:2;">
*[[Common chemicals]] - Where to find common chemical components
*[[List of basic chemistry topics]]
*[[List of chemistry topics]]
*[[List of chemists]]
*[[List of compounds]]
*[[List of important publications in chemistry]]
*[[Periodic Table of the Elements]]
*[[Timeline of chemistry]]
*[[Unsolved problems in chemistry]]
</div>

==External links==
*[http://www.iupac.org/dhtml_home.html International Union of Pure and Applied Chemistry]
*[http://www.chem.qmw.ac.uk/iupac/ IUPAC Nomenclature Home Page], see especially the "Gold Book" containing definitions of standard chemical terms
*[http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/chemcon.html#c1 Interactive [[Mind Map]] of Chemistry]
*[http://www.smallscalechemistry.colostate.edu/PowerfulPictures/ChemicalEnergetics.pdf / Chemical energetics]

{{BranchesofChemistry}}

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==References==
{{reflist}}

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