Editing Chemistry (section)

==Modern principles==
[[File:Lab bench.jpg|thumb|upright=1.15|[[Laboratory]], Institute of Biochemistry, [[University of Cologne]] in [[Germany]]]]

The current model of atomic structure is the [[quantum mechanical model]].<ref>{{cite encyclopedia|title=chemical bonding|url=https://www.britannica.com/EBchecked/topic/684121/chemical-bonding/43383/The-quantum-mechanical-model|encyclopedia=Britannica|publisher=Encyclopædia Britannica|access-date=1 November 2012|archive-date=26 April 2012|archive-url=https://web.archive.org/web/20120426054956/http://www.britannica.com/EBchecked/topic/684121/chemical-bonding/43383/The-quantum-mechanical-model|url-status=live}}</ref> Traditional chemistry starts with the study of [[elementary particles]], [[atom]]s, [[molecule]]s,<ref>Anthony Carpi. [http://www.visionlearning.com/library/module_viewer.php?mid=49 Matter: Atoms from Democritus to Dalton], {{Webarchive|url=https://web.archive.org/web/20070228203304/http://www.visionlearning.com/library/module_viewer.php?mid=49|date=28 February 2007}}.</ref> [[chemical substance|substances]], [[metal]]s, [[crystal]]s and other aggregates of [[matter]]. Matter can be studied in solid, liquid, gas and plasma [[states of matter|states]], in isolation or in combination. The interactions, [[Chemical reaction|reactions]] and transformations that are studied in chemistry are usually the result of interactions between atoms, leading to rearrangements of the [[chemical bond]]s which hold atoms together. Such behaviors are studied in a chemistry [[laboratory]].

The chemistry laboratory stereotypically uses various forms of [[laboratory glassware]]. However glassware is not central to chemistry, and a great deal of experimental (as well as applied/industrial) chemistry is done without it.

[[File:Chemicals in flasks.jpg|thumb|right|Solutions of substances in reagent bottles, including [[ammonium hydroxide]] and [[nitric acid]], illuminated in different colors]]

A [[chemical reaction]] is a transformation of some substances into one or more different substances.<ref>IUPAC, [[Gold Book]] [http://goldbook.iupac.org/C01033.html Definition], {{Webarchive|url=https://web.archive.org/web/20070304035235/http://goldbook.iupac.org/C01033.html|date=4 March 2007}}.</ref> The basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. It can be symbolically depicted through a [[chemical equation]], which usually involves atoms as subjects. The number of atoms on the left and the right in the equation for a chemical transformation is equal. (When the number of atoms on either side is unequal, the transformation is referred to as a [[nuclear reaction]] or [[radioactive decay]].) The type of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as [[chemical law]]s.

[[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 analyzed using the tools of [[chemical analysis]], e.g. [[spectroscopy]] and [[chromatography]]. Scientists engaged in chemical research are known as [[chemists]].<ref>{{cite web |url=http://www.calmis.ca.gov/file/occguide/CHEMIST.HTM |title=California Occupational Guide Number 22: Chemists |publisher=Calmis.ca.gov |date=29 October 1999 |access-date=12 June 2011 |archive-url=https://web.archive.org/web/20110610111332/http://www.calmis.ca.gov/file/occguide/CHEMIST.HTM |archive-date=10 June 2011 |url-status=dead }}</ref> Most chemists specialize in one or more sub-disciplines. Several [[concept]]s are essential for the study of chemistry; some of them are:<ref>{{cite web |url=http://antoine.frostburg.edu/chem/senese/101/matter/ |title=General Chemistry Online – Companion Notes: Matter |publisher=Antoine.frostburg.edu |access-date=12 June 2011 |archive-date=24 June 2011 |archive-url=https://web.archive.org/web/20110624140458/http://antoine.frostburg.edu/chem/senese/101/matter/ |url-status=live }}</ref>

===Matter===
{{Main|Matter}}
In chemistry, matter is defined as anything that has [[invariant mass|rest mass]] and [[volume]] (it takes up space) and is made up of [[particle]]s. The particles that make up matter have rest mass as well – not all particles have rest mass, such as the [[photon]]. Matter can be a pure [[chemical substance]] or a [[mixture]] of substances.<ref>{{cite book |last=Armstrong |first=James |title=General, Organic, and Biochemistry: An Applied Approach |publisher=[[Brooks/Cole]] |year=2012 |isbn=978-0-534-49349-3 |page=48}}</ref>

====Atom====
{{Main|Atom}}
[[File:Atom Diagram.svg|thumb|upright=0.75|left|A diagram of an atom based on the [[Rutherford model]]]]

The [[atom]] is the basic unit of chemistry. It consists of a dense core called the [[atomic nucleus]] surrounded by a space occupied by an [[electron cloud]]. The nucleus is made up of positively charged [[protons]] and uncharged [[neutrons]] (together called [[nucleon]]s), while the [[electron]] cloud consists of negatively charged [[electron]]s which orbit the nucleus. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons. The nucleus is dense; the mass of a nucleon is approximately 1,836 times that of an electron, yet the radius of an atom is about 10,000 times that of its nucleus.{{sfn|Burrows|Holman|Parsons|Pilling|2009|p=13}}{{sfn|Housecroft|Sharpe|2008|p=2}}

The atom is also the smallest entity that can be envisaged to retain the [[chemical properties]] of the element, such as [[electronegativity]], [[ionization potential]], preferred [[oxidation state]](s), [[coordination number]], and preferred types of bonds to form (e.g., [[metal]]lic, [[ion]]ic, [[covalent]]).

====Element====
[[File:Simple Periodic Table Chart-blocks.svg|thumb|right|upright=1.35|Standard form of the [[periodic table]] of chemical elements. The colors represent different [[block (periodic table)|blocks]] of elements.]]
{{Main|Chemical element}}

A chemical element is a pure substance which is composed of a single type of atom, characterized by its particular number of [[proton]]s in the nuclei of its atoms, known as the [[atomic number]] and represented by the symbol ''Z''. The [[mass number]] is the sum of the number of protons and neutrons in a nucleus. Although all the nuclei of all atoms belonging to one element will have the same atomic number, they may not necessarily have the same mass number; atoms of an element which have different mass numbers are known as [[isotope]]s. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element [[carbon]], but atoms of carbon may have mass numbers of 12 or 13.{{sfn|Housecroft|Sharpe|2008|p=2}}

The standard presentation of the chemical elements is in the [[periodic table]], which orders elements by atomic number. The periodic table is arranged in [[Periodic table group|groups]], or columns, and [[period (periodic table)|periods]], or rows. The periodic table is useful in identifying [[periodic trends]].{{sfn|Burrows|Holman|Parsons|Pilling|2009|p=110}}

====Compound====
[[File:Carbon dioxide structure.png|thumb|left|upright=0.6|[[Carbon dioxide]] (CO<sub>2</sub>), an example of a chemical compound]]
{{Main|Chemical compound}}

A ''compound'' is a pure chemical substance composed of more than one element. The properties of a compound bear little similarity to those of its elements.{{sfn|Burrows|Holman|Parsons|Pilling|2009|p=12}} The standard nomenclature of compounds is set by the [[International Union of Pure and Applied Chemistry]] (IUPAC). [[Organic compound]]s are named according to the [[organic nomenclature]] system.<ref>{{cite web |url=http://www.acdlabs.com/iupac/nomenclature/ |title=IUPAC Nomenclature of Organic Chemistry |publisher=Acdlabs.com |access-date=12 June 2011 |archive-date=8 June 2011 |archive-url=https://web.archive.org/web/20110608140820/http://www.acdlabs.com/iupac/nomenclature/ |url-status=live }}</ref> The names for [[inorganic compound]]s are created according to the [[inorganic nomenclature]] system. When a compound has more than one component, then they are divided into two classes, the electropositive and the electronegative components.<ref name="IUPAC">{{cite book |last1=Connelly |first1=Neil G. |last2=Damhus |first2=Ture |last3=Hartshom |first3=Richard M. |last4=Hutton |first4=Alan T. |title=Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005. |date=2005 |publisher=Royal Society of Chemistry Publishing / IUPAC |location=Cambridge |isbn=0854044388 |url=https://archive.org/details/nomenclatureinor2005conn |access-date=13 June 2022}}</ref> In addition the [[Chemical Abstracts Service|Chemical Abstracts Service (CAS)]] has devised a method to index chemical substances. In this scheme each chemical substance is identifiable by a number known as its [[CAS registry number]].

====Molecule====
{{Main|Molecule}}
[[File:Caffeine (1) 3D ball.png|upright=1.05|thumb|right|A ball-and-stick representation of the [[caffeine]] molecule (C<sub>8</sub>H<sub>10</sub>N<sub>4</sub>O<sub>2</sub>)]]

A ''molecule'' is the smallest indivisible portion 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. However, this definition only works well for substances that are composed of molecules, which is not true of many substances (see below). 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.

Thus, molecules exist as electrically neutral units, unlike ions. When this rule is broken, giving the "molecule" a charge, the result is sometimes named a [[molecular ion]] or a polyatomic ion. However, the discrete and separate nature of the molecular concept usually requires that molecular ions be present only in well-separated form, such as a directed beam in a vacuum in a [[mass spectrometer]]. Charged polyatomic collections residing in solids (for example, common [[sulfate]] or [[nitrate]] ions) are generally not considered "molecules" in chemistry. Some molecules contain one or more unpaired electrons, creating [[Radical (chemistry)|radicals]]. Most radicals are comparatively reactive, but some, such as [[nitric oxide]] (NO) can be stable.

[[File:Benzene-2D-full.svg|thumb|upright=0.7|left|A 2-D [[structural formula]] of a [[benzene]] molecule (C<sub>6</sub>H<sub>6</sub>)]]
The "inert" or [[Noble gas|noble gas elements]] ([[helium]], [[neon]], [[argon]], [[krypton]], [[xenon]] and [[radon]]) are composed of lone atoms as their smallest discrete unit, but the other isolated chemical elements consist of either molecules or networks of atoms bonded to each other in some way. Identifiable molecules compose familiar substances such as water, air, and many organic compounds like alcohol, sugar, gasoline, and the various [[pharmaceutical]]s.

However, not all substances or chemical compounds consist of discrete molecules, and indeed most of the solid substances that make up the solid crust, mantle, and core of the Earth are chemical compounds without molecules. These other types of substances, such as [[ionic compounds]] and [[network solids]], are organized in such a way as to lack the existence of identifiable molecules ''per se''. Instead, these substances are discussed in terms of [[formula unit]]s or [[unit cell]]s as the smallest repeating structure within the substance. Examples of such substances are mineral salts (such as [[table salt]]), solids like carbon and diamond, metals, and familiar [[silica]] and [[silicate minerals]] such as quartz and granite.

One of the main characteristics 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 molecular geometry|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.

====Substance and mixture====
{{infobox
| data1 = [[File:Cín.png|100px]] [[File:Sulfur-sample.jpg|100px]]
| data2 = [[File:Diamants maclés 2(République d'Afrique du Sud).jpg|100px]] [[File:Sugar 2xmacro.jpg|100px]]
| data3 = [[File:Sal (close).jpg|100px]] [[File:Sodium bicarbonate.jpg|100px]]
| data5 = Examples of pure chemical substances. From left to right: the elements [[tin]] (Sn) and [[sulfur]] (S), [[diamond]] (an [[allotrope]] of [[carbon]]), [[sucrose]] (pure sugar), and [[sodium chloride]] (salt) and [[sodium bicarbonate]] (baking soda), which are both ionic compounds.
}}
A chemical substance is a kind of matter with a definite [[chemical composition|composition]] and set of [[chemical properties|properties]].<ref>{{Cite book |last1=Hill |first1=J. W. |title=General Chemistry |last2=Petrucci |first2=R. H. |last3=McCreary |first3=T. W. |last4=Perry |first4=S. S. |publisher=Pearson Prentice Hall |year=2005 |edition=4th |location=Upper Saddle River, New Jersey |page=37}}</ref> A collection of substances is called a mixture. Examples of mixtures are [[Earth's atmosphere|air]] and [[alloy]]s.<ref>{{cite book |last1=Avedesian |first1=M. M. |title=Magnesium and Magnesium Alloys |last2=Baker |first2=Hugh |publisher=ASM International |page=59}}</ref>

====Mole and amount of substance====
{{Main|Mole (unit)|l1=Mole}}
The mole is a unit of measurement that denotes an [[amount of substance]] (also called chemical amount). One mole is defined to contain exactly {{val|6.02214076|e=23}} particles (atoms, molecules, ions, or electrons), where the [[particle number|number of particles]] per mole is known as the [[Avogadro constant]].{{sfn|Burrows|Holman|Parsons|Pilling|2009|p=16}} [[Molar concentration]] is the amount of a particular substance per volume of [[Solution (chemistry)|solution]], and is commonly reported in mol/[[decimetre|dm]]<sup>3</sup>.{{sfn|Atkins|de Paula|2009|p=9}}

===Phase===
[[File:Phase changes.svg|thumb|upright=1.25|Diagram showing relationships among the phases and the terms used to describe phase changes]]
{{Main|Phase (matter)|l1=Phase}}

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. Another phase commonly encountered in the study of chemistry is the ''aqueous'' phase, which is the state of substances dissolved in [[aqueous solution]] (that is, in water).

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]].

===Bonding===
{{Main|Chemical bond}}
[[File:Ionic bonding animation.gif|thumb|right|upright=1.15|An animation of the process of ionic bonding between [[sodium]] (Na) and [[chlorine]] (Cl) to form [[sodium chloride]], or common table salt. Ionic bonding involves one atom taking valence electrons from another (as opposed to sharing, which occurs in covalent bonding).]]

Atoms sticking together in molecules or crystals are said to be bonded with one another. A chemical bond may be visualized as the [[multipole]] balance between the positive charges in the nuclei and the negative charges oscillating about them.<ref>{{cite web |author= |title=Chemical Bonding by Anthony Carpi, PhD |url=http://www.visionlearning.com/library/module_viewer.php?mid=55 |url-status=live |archive-url=https://web.archive.org/web/20110717215216/http://www.visionlearning.com/library/module_viewer.php?mid=55 |archive-date=17 July 2011 |access-date=12 June 2011 |publisher=visionlearning}}</ref> More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom.

The chemical bond can be a [[covalent bond]], an [[ionic bond]], a [[hydrogen bond]] or just because of [[Van der Waals force]]. Each of these kinds of bonds is ascribed to some potential. These potentials create the interactions which hold atoms together 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 explain molecular structure and composition.

An ionic bond is formed when a metal loses one or more of its electrons, becoming a positively charged cation, and the electrons are then gained by the non-metal atom, becoming a negatively charged anion. The two oppositely charged ions attract one another, and the ionic bond is the electrostatic force of attraction between them. For example, [[sodium]] (Na), a metal, loses one electron to become an Na<sup>+</sup> cation while [[chlorine]] (Cl), a non-metal, gains this electron to become [[Chloride|Cl<sup>−</sup>]]. The ions are held together due to electrostatic attraction, and that compound [[sodium chloride]] (NaCl), or common table salt, is formed.

[[File:Elektronenformel Punkte CH4.svg|thumb|upright=0.75|left|In the [[methane]] molecule (CH<sub>4</sub>), the carbon atom shares a pair of valence electrons with each of the four hydrogen atoms. Thus, the octet rule is satisfied for C-atom (it has eight electrons in its valence shell) and the duet rule is satisfied for the H-atoms (they have two electrons in their valence shells).]]
In a covalent bond, one or more pairs of [[valence electron]]s are shared by two atoms: the resulting electrically neutral group of bonded atoms is termed a [[molecule]]. Atoms will share valence electrons in such a way as to create a [[noble gas]] electron configuration (eight electrons in their outermost shell) for each atom. Atoms that tend to combine in such a way that they each have eight electrons in their valence shell are said to follow the [[octet rule]]. However, some elements like [[hydrogen]] and [[lithium]] need only two electrons in their outermost shell to attain this stable configuration; these atoms are said to follow the ''duet rule'', and in this way they are reaching the electron configuration of the noble gas [[helium]], which has two electrons in its outer shell.

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 is less applicable and alternative approaches, such as the [[molecular orbital]] theory, are generally used.

===Energy===
{{Main|Energy}}

In the context of chemistry, energy is an attribute of a substance as a consequence of its [[atomic structure|atomic]], [[molecular structure|molecular]] or aggregate [[chemical structure|structure]]. Since a chemical transformation is accompanied by a change in one or more of these kinds of structures, it 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 [[exergonic reaction|exergonic]] if the final state is lower on the energy scale than the initial state; in the case of [[endergonic reaction]]s the situation is the reverse. A reaction is said to be [[exothermic reaction|exothermic]] if the reaction releases heat to the surroundings; in the case of [[endothermic reaction]]s, the reaction absorbs heat from the surroundings.

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 a 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 to occur can be in the form of heat, light, [[electricity]] or mechanical [[force]] in the form of [[ultrasound]].<ref>Reilly, Michael. (2007). [https://www.newscientist.com/article/dn11427-mechanical-force-induces-chemical-reaction.html Mechanical force induces chemical reaction], {{Webarchive|url=https://web.archive.org/web/20140814004108/http://www.newscientist.com/article/dn11427-mechanical-force-induces-chemical-reaction.html#.Uy6ySlendfA|date=14 August 2014}}, NewScientist.com news service.</ref>

A related concept [[thermodynamic free energy|free energy]], which also incorporates entropy considerations, 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 exist only 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 a 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, more amenable to chemical reactions.

The phase of a substance is invariably determined by its energy and the energy of its surroundings. When the [[intermolecular force]]s of a substance are such that the 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], {{Webarchive|url=https://web.archive.org/web/20070428171905/http://www.chem4kids.com/files/matter_changes.html|date=28 April 2007}}, 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 [[dipole–dipole interaction]]s.

The transfer of energy from one chemical substance to another depends on the ''size'' of energy [[quantum|quanta]] emitted from one substance. However, heat energy is often transferred more easily from almost any substance to another because the [[phonons]] responsible for vibrational and rotational energy levels in a substance have much less energy than [[photons]] invoked for the electronic energy transfer. Thus, because vibrational and rotational energy levels are more closely spaced than electronic energy levels, heat is more easily transferred between substances relative to light or other forms of electronic energy. For example, [[ultraviolet]] electromagnetic radiation is not transferred with as much efficacy from one substance to another as thermal or electrical energy.

The existence of characteristic energy levels for different [[chemical substance]]s is useful for their identification by the analysis of [[spectral lines]]. Different kinds of spectra are often used in chemical [[spectroscopy]], e.g. [[infrared spectroscopy|IR]], [[microwave spectroscopy|microwave]], [[NMR]], [[electron spin resonance|ESR]], etc. Spectroscopy is also used to identify the composition of remote objects – like stars and distant galaxies – by analyzing their radiation spectra.
[[File:Emission spectrum-Fe.svg|thumb|upright=2.2|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 to transform other chemical substances.

===Reaction===
{{Main|Chemical reaction}}
[[File:VysokePece1.jpg|thumb|right|During chemical reactions, bonds between atoms break and form, resulting in different substances with different properties. In a blast furnace, iron oxide, a [[chemical compound|compound]], reacts with carbon monoxide to form iron, one of the [[chemical element]]s, and carbon dioxide.]]
When a chemical substance is transformed as a result of its interaction with another substance or with energy, a chemical reaction is said to have occurred. A ''chemical reaction'' is therefore a concept related to the "reaction" of a substance when it comes in close contact with another, whether as a mixture or a [[Solution (chemistry)|solution]]; exposure to some form of energy, or both. It results in some energy exchange between the constituents of the reaction as well as with the system environment, which may be designed vessels—often [[laboratory glassware]].

Chemical reactions can result in the formation or [[dissociation (chemistry)|dissociation]] of molecules, that is, molecules breaking apart to form two or more molecules or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds. [[Redox|Oxidation, reduction]], [[dissociation (chemistry)|dissociation]], acid–base [[neutralization (chemistry)|neutralization]] and molecular [[rearrangement reaction|rearrangement]] are some examples of common 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], {{Webarchive|url=https://web.archive.org/web/20071012013002/http://goldbook.iupac.org/C01034.html|date=12 October 2007}}, 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 in handy while proposing a mechanism for a chemical reaction.

According to the [[IUPAC]] gold book, 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], {{Webarchive|url=https://web.archive.org/web/20070304035235/http://goldbook.iupac.org/C01033.html|date=4 March 2007}}, IUPAC Goldbook.</ref> Accordingly, 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 [[Conformational isomerism|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').

===Ions and salts===
[[File:Potassium-chloride-3D-ionic.png|thumb|upright=0.75|The crystal lattice structure of [[potassium chloride]] (KCl), a salt which is formed due to the attraction of K<sup>+</sup> cations and Cl<sup>−</sup> anions. The overall charge of the ionic compound is zero.]]
{{Main|Ion}}

An ''ion'' is a charged species, an atom or a molecule, that has lost or gained one or more electrons. When an atom loses an electron and thus has more protons than electrons, the atom is a positively charged ion or [[cation]]. When an atom gains an electron and thus has more electrons than protons, the atom is a negatively charged ion or [[anion]]. Cations and anions can form a crystalline lattice of neutral [[salt (chemistry)|salts]], such as the Na<sup>+</sup> and Cl<sup>−</sup> ions forming [[sodium chloride]], or NaCl. Examples of [[polyatomic ion]]s that do not split up during [[acid–base reaction]]s are [[hydroxide]] (OH<sup>−</sup>) and [[phosphate]] (PO<sub>4</sub><sup>3−</sup>).

[[plasma (physics)|Plasma]] is composed of gaseous matter that has been completely ionized, usually through high temperature.

===Acidity and basicity===
[[File:Hydrogen-bromide-3D-vdW.svg|thumb|left|upright=0.8|[[Hydrogen bromide]] exists in the gas phase as a diatomic molecule.]]
{{Main|Acid–base reaction}}

A substance can often be classified as an [[acid]] or a [[base (chemistry)|base]]. There are several different theories which explain acid–base behavior. The simplest is [[Arrhenius acid|Arrhenius theory]], which states that an acid is a substance that produces [[hydronium ion]]s when it is dissolved in water, and a base is one that produces [[hydroxide ion]]s when dissolved in water. According to [[Brønsted–Lowry acid–base theory]], acids are substances that donate a positive [[hydrogen]] [[ion]] to another substance in a chemical reaction; by extension, a base is the substance which receives that hydrogen ion.

A third common theory is [[Lewis acids and bases|Lewis acid–base theory]], which is based on the formation of new chemical bonds. Lewis theory explains that an acid is a substance which is capable of accepting a pair of electrons from another substance during the process of bond formation, while a base is a substance which can provide a pair of electrons to form a new bond. There are several other ways in which a substance may be classified as an acid or a base, as is evident in the history of this concept.<ref>{{cite web |url=https://www.bbc.co.uk/dna/h2g2/A708257 |title=History of Acidity |publisher=BBC |date=27 May 2004 |access-date=12 June 2011 |archive-date=27 February 2009 |archive-url=https://web.archive.org/web/20090227024430/http://www.bbc.co.uk/dna/h2g2/A708257 |url-status=live }}</ref>

Acid strength is commonly measured by two methods. One measurement, based on the Arrhenius definition of acidity, is [[pH]], which is a measurement of the hydronium ion concentration in a solution, as expressed on a negative [[logarithm]]ic scale. Thus, solutions that have a low pH have a high hydronium ion concentration and can be said to be more acidic. The other measurement, based on the Brønsted–Lowry definition, is the [[acid dissociation constant]] (K<sub>a</sub>), which measures the relative ability of a substance to act as an acid under the Brønsted–Lowry definition of an acid. That is, substances with a higher K<sub>a</sub> are more likely to donate hydrogen ions in chemical reactions than those with lower K<sub>a</sub> values.

===Redox===
{{Main|Redox}}

Redox ({{not a typo|{{em|red}}uction}}-{{not a typo|{{em|ox}}idation}}) reactions include all [[chemical reaction]]s in which atoms have their [[oxidation state]] changed by either gaining electrons (reduction) or losing electrons (oxidation). Substances that have the ability to oxidize other substances are said to be oxidative and are known as [[oxidizing agents]], oxidants or oxidizers. An oxidant removes electrons from another substance. Similarly, substances that have the ability to reduce other substances are said to be reductive and are known as [[reducing agents]], reductants, or reducers.

A reductant transfers electrons to another substance and is thus oxidized itself. And because it "donates" electrons it is also called an electron donor. Oxidation and reduction properly refer to a change in oxidation number—the actual transfer of electrons may never occur. Thus, oxidation is better defined as an increase in [[oxidation number]], and reduction as a decrease in oxidation number.

===Equilibrium===
{{Main|Chemical equilibrium}}

Although the concept of [[Chemical equilibrium|equilibrium]] is widely used across sciences, in the context of chemistry, it arises whenever a number of different states of the chemical composition are possible, as for example, in a mixture of several chemical compounds that can react with one another, or when a substance can be present in more than one kind of phase.

A system of chemical substances at equilibrium, even though having an unchanging composition, is most often not [[static equilibrium|static]]; molecules of the substances continue to react with one another thus giving rise to a [[dynamic equilibrium]]. Thus the concept describes the state in which the parameters such as chemical composition remain unchanged over time.

===Chemical laws===
{{Main|Chemical law}}

Chemical reactions are governed by certain laws, which have become fundamental concepts in chemistry. Some of them are:
{{div col|colwidth=30em}}
* [[Avogadro's law]]
* [[Beer–Lambert law]]
* [[Boyle's law]] (1662, relating pressure and volume)
* [[Charles's law]] (1787, relating volume and temperature)
* [[Fick's laws of diffusion]]
* [[Pressure law|Gay-Lussac's law]] (1809, relating pressure and temperature)
* [[Le Chatelier's principle]]
* [[Henry's law]]
* [[Hess's law]]
* [[Conservation of energy|Law of conservation of energy]] leads to the important concepts of [[Chemical equilibrium|equilibrium]], [[thermodynamics]], and [[chemical kinetics|kinetics]].
* [[Law of conservation of mass]] continues to be conserved in [[isolated system]]s, even in modern physics. However, [[special relativity]] shows that due to [[mass–energy equivalence]], whenever non-material "energy" (heat, light, kinetic energy) is removed from a non-isolated system, some mass will be lost with it. High energy losses result in loss of weighable amounts of mass, an important topic in [[nuclear chemistry]].
* [[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]]
* [[Raoult's law]]
{{div col end}}