Chemistry is the scientific study of the properties and behavior ofmatter.^[1] It is aphysical science within thenatural sciences that studies thechemical elements that make up matter andcompounds made ofatoms,molecules andions: their composition, structure, properties, behavior and the changes they undergo duringreactions with othersubstances.^[2][3][4][5] Chemistry also addresses the nature ofchemical bonds inchemical compounds.

In the scope of its subject, chemistry occupies an intermediate position betweenphysics andbiology.^[6] It is sometimes calledthe central science because it provides a foundation for understanding bothbasic andapplied scientific disciplines at a fundamental level.^[7] For example, chemistry explains aspects of plant growth (botany), the formation of igneous rocks (geology), how atmospheric ozone is formed and how environmental pollutants are degraded (ecology), the properties of the soil on the Moon (cosmochemistry), how medications work (pharmacology), and how to collectDNA evidence at a crime scene (forensics).

Chemistry has existed under various names since ancient times.^[8] It has evolved, and now chemistry encompasses various areas of specialisation, or subdisciplines, that continue to increase in number and interrelate to create further interdisciplinary fields of study. The applications of various fields of chemistry are used frequently for economic purposes in thechemical industry.

The wordchemistry comes from a modification during theRenaissance of the wordalchemy, which referred to an earlier set of practices that encompassed elements of chemistry,metallurgy,philosophy,astrology,astronomy,mysticism, andmedicine. Alchemy is often associated with the quest to turn lead or other base metals into gold, thoughalchemists were also interested in many of the questions of modern chemistry.^[9][10]

The modern wordalchemy in turn is derived from theArabic wordal-kīmīā (الكیمیاء). This may haveEgyptian origins sinceal-kīmīā is derived from theAncient Greekχημία, which is in turn derived from the wordKemet, which is the ancient name of Egypt in the Egyptian language.^[11] Alternately,al-kīmīā may derive fromχημεία 'cast together'.^[12]

The current model of atomic structure is thequantum mechanical model.^[13] Traditional chemistry starts with the study ofelementary particles,atoms,molecules,^[14]substances,metals,crystals and other aggregates ofmatter. Matter can be studied in solid, liquid, gas and plasmastates, in isolation or in combination. The interactions,reactions and transformations that are studied in chemistry are usually the result of interactions between atoms, leading to rearrangements of thechemical bonds which hold atoms together. Such behaviors are studied in a chemistrylaboratory.

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

Achemical reaction is a transformation of some substances into one or more different substances.^[15] The basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. It can be symbolically depicted through achemical 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 anuclear reaction orradioactive 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 aschemical laws.

Energy andentropy considerations are invariably important in almost all chemical studies. Chemical substances are classified in terms of theirstructure, phase, as well as theirchemical compositions. They can be analyzed using the tools ofchemical analysis, e.g.spectroscopy andchromatography. Scientists engaged in chemical research are known aschemists.^[16] Most chemists specialize in one or more sub-disciplines. Severalconcepts are essential for the study of chemistry; some of them are:^[17]

In chemistry, matter is defined as anything that hasrest mass andvolume (it takes up space) and is made up ofparticles. The particles that make up matter have rest mass as well – not all particles have rest mass, such as thephoton. Matter can be a purechemical substance or amixture of substances.^[18]

Theatom is the basic unit of chemistry. It consists of a dense core called theatomic nucleus surrounded by a space occupied by anelectron cloud. The nucleus is made up of positively chargedprotons and unchargedneutrons (together callednucleons), while theelectron cloud consists of negatively chargedelectrons 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.^[19][20]

The atom is also the smallest entity that can be envisaged to retain thechemical properties of the element, such aselectronegativity,ionization potential, preferredoxidation state(s),coordination number, and preferred types of bonds to form (e.g.,metallic,ionic,covalent).

A chemical element is a pure substance which is composed of a single type of atom, characterized by its particular number ofprotons in the nuclei of its atoms, known as theatomic number and represented by the symbolZ. Themass 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 asisotopes. For example, all atoms with 6 protons in their nuclei are atoms of the chemical elementcarbon, but atoms of carbon may have mass numbers of 12 or 13.^[20]

The standard presentation of the chemical elements is in theperiodic table, which orders elements by atomic number. The periodic table is arranged ingroups, or columns, andperiods, or rows. The periodic table is useful in identifyingperiodic trends.^[21]

Acompound is a pure chemical substance composed of more than one element. The properties of a compound bear little similarity to those of its elements.^[22] The standard nomenclature of compounds is set by theInternational Union of Pure and Applied Chemistry (IUPAC).Organic compounds are named according to theorganic nomenclature system.^[23] The names forinorganic compounds are created according to theinorganic nomenclature system. When a compound has more than one component, then they are divided into two classes, the electropositive and the electronegative components.^[24] In addition theChemical Abstracts Service has devised a method to index chemical substances. In this scheme each chemical substance is identifiable by a number known as itsCAS registry number.

Amolecule is the smallest indivisible portion of a purechemical 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 bycovalent bonds, such that the structure is electrically neutral and all valence electrons are paired with other electrons either in bonds or inlone pairs.

Thus, molecules exist as electrically neutral units, unlike ions. When this rule is broken, giving the "molecule" a charge, the result is sometimes named amolecular 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 amass spectrometer. Charged polyatomic collections residing in solids (for example, commonsulfate ornitrate ions) are generally not considered "molecules" in chemistry. Some molecules contain one or more unpaired electrons, creatingradicals. Most radicals are comparatively reactive, but some, such asnitric oxide (NO) can be stable.

The "inert" ornoble gas elements (helium,neon,argon,krypton,xenon andradon) 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 variouspharmaceuticals.

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 asionic compounds andnetwork solids, are organized in such a way as to lack the existence of identifiable moleculesper se. Instead, these substances are discussed in terms offormula units orunit cells as the smallest repeating structure within the substance. Examples of such substances are mineral salts (such astable salt), solids like carbon and diamond, metals, and familiarsilica andsilicate minerals such as quartz and granite.

One of the main characteristics of a molecule is its geometry often called itsstructure. While the structure ofdiatomic,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.

A chemical substance is a kind of matter with a definitecomposition and set ofproperties.^[25] A collection of substances is called a mixture. Examples of mixtures areair andalloys.^[26]

The mole is a unit of measurement that denotes anamount of substance (also called chemical amount). One mole is defined to contain exactly6.02214076×10²³ particles (atoms, molecules, ions, or electrons), where thenumber of particles per mole is known as theAvogadro constant.^[27]Molar concentration is the amount of a particular substance per volume ofsolution, and is commonly reported in mol/dm³.^[28]

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. Aphase is a set of states of a chemical system that have similar bulk structural properties, over a range of conditions, such aspressure ortemperature.

Physical properties, such asdensity andrefractive index tend to fall within values characteristic of the phase. The phase of matter is defined by thephase 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 asupercritical state. When three states meet based on the conditions, it is known as atriple point and since this is invariant, it is a convenient way to define a set of conditions.

The most familiar examples of phases aresolids,liquids, andgases. Many substances exhibit multiple solid phases. For example, there are three phases of solidiron (alpha, gamma, and delta) that vary based on temperature and pressure. A principal difference between solid phases is thecrystal structure, or arrangement, of the atoms. Another phase commonly encountered in the study of chemistry is theaqueous phase, which is the state of substances dissolved inaqueous solution (that is, in water).

Less familiar phases includeplasmas,Bose–Einstein condensates andfermionic condensates and theparamagnetic andferromagnetic phases ofmagnetic 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 inbiology.

Atoms sticking together in molecules or crystals are said to be bonded with one another. A chemical bond may be visualized as themultipole balance between the positive charges in the nuclei and the negative charges oscillating about them.^[29] 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 acovalent bond, anionic bond, ahydrogen bond or just because ofVan der Waals force. Each of these kinds of bonds is ascribed to some potential. These potentials create the interactions which hold atoms together inmolecules orcrystals. In many simple compounds,valence bond theory, the Valence Shell Electron Pair Repulsion model (VSEPR), and the concept ofoxidation 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⁺ cation whilechlorine (Cl), a non-metal, gains this electron to becomeCl⁻. The ions are held together due to electrostatic attraction, and that compoundsodium chloride (NaCl), or common table salt, is formed.

In a covalent bond, one or more pairs ofvalence electrons are shared by two atoms: the resulting electrically neutral group of bonded atoms is termed amolecule. Atoms will share valence electrons in such a way as to create anoble 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 theoctet rule. However, some elements likehydrogen andlithium need only two electrons in their outermost shell to attain this stable configuration; these atoms are said to follow theduet rule, and in this way they are reaching the electron configuration of the noble gashelium, which has two electrons in its outer shell.

Similarly, theories fromclassical physics can be used to predict many ionic structures. With more complicated compounds, such asmetal complexes, valence bond theory is less applicable and alternative approaches, such as themolecular orbital theory, are generally used.

In the context of chemistry, energy is an attribute of a substance as a consequence of itsatomic,molecular or aggregatestructure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structures, it is invariably accompanied by anincrease ordecrease ofenergy of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of heat orlight; thus the products of a reaction may have more or less energy than the reactants.

A reaction is said to beexergonic if the final state is lower on the energy scale than the initial state; in the case ofendergonic reactions the situation is the reverse. A reaction is said to beexothermic if the reaction releases heat to the surroundings; in the case ofendothermic reactions, the reaction absorbs heat from the surroundings.

Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as theactivation energy. Thespeed of a chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor${\displaystyle e^{-E/kT}}$  – 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 theArrhenius equation.The activation energy necessary for a chemical reaction to occur can be in the form of heat, light,electricity or mechanicalforce in the form ofultrasound.^[30]

A related conceptfree 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, inchemical thermodynamics. A reaction is feasible only if the total change in theGibbs free energy is negative,${\displaystyle \mathrm{\Delta }G\le 0}$ ; if it is equal to zero the chemical reaction is said to be atequilibrium.

There exist only limited possible states of energy for electrons, atoms and molecules. These are determined by the rules ofquantum mechanics, which requirequantization 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 theintermolecular forces 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 (H₂O); a liquid at room temperature because its molecules are bound byhydrogen bonds.^[31] Whereashydrogen sulfide (H₂S) is a gas at room temperature and standard pressure, as its molecules are bound by weakerdipole–dipole interactions.

The transfer of energy from one chemical substance to another depends on thesize of energyquanta emitted from one substance. However, heat energy is often transferred more easily from almost any substance to another because thephonons responsible for vibrational and rotational energy levels in a substance have much less energy thanphotons 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 differentchemical substances is useful for their identification by the analysis ofspectral lines. Different kinds of spectra are often used in chemicalspectroscopy, e.g.IR,microwave,NMR,ESR, etc. Spectroscopy is also used to identify the composition of remote objects – like stars and distant galaxies – by analyzing their radiation spectra.

The termchemical energy is often used to indicate the potential of a chemical substance to undergo a transformation through achemical reaction or to transform other chemical substances.

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. Achemical 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 asolution; 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—oftenlaboratory glassware.

Chemical reactions can result in the formation ordissociation 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.Oxidation, reduction,dissociation, acid–baseneutralization and molecularrearrangement are some examples of common chemical reactions.

A chemical reaction can be symbolically depicted through achemical 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.^[32]

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

According to theIUPAC gold book, a chemical reaction is "a process that results in the interconversion of chemical species."^[33] Accordingly, a chemical reaction may be anelementary reaction or astepwise reaction. An additional caveat is made, in that this definition includes cases where theinterconversion 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').

Anion 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 orcation. When an atom gains an electron and thus has more electrons than protons, the atom is a negatively charged ion oranion. Cations and anions can form a crystalline lattice of neutralsalts, such as the Na⁺ and Cl⁻ ions formingsodium chloride, or NaCl. Examples ofpolyatomic ions that do not split up duringacid–base reactions arehydroxide (OH⁻) andphosphate (PO₄³⁻).

Plasma is composed of gaseous matter that has been completely ionized, usually through high temperature.

A substance can often be classified as anacid or abase. There are several different theories which explain acid–base behavior. The simplest isArrhenius theory, which states that an acid is a substance that produceshydronium ions when it is dissolved in water, and a base is one that produceshydroxide ions when dissolved in water. According toBrønsted–Lowry acid–base theory, acids are substances that donate a positivehydrogenion to another substance in a chemical reaction; by extension, a base is the substance which receives that hydrogen ion.

A third common theory isLewis 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.^[34]

Acid strength is commonly measured by two methods. One measurement, based on the Arrhenius definition of acidity, ispH, which is a measurement of the hydronium ion concentration in a solution, as expressed on a negativelogarithmic 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 theacid dissociation constant (K_a), 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_a are more likely to donate hydrogen ions in chemical reactions than those with lower K_a values.

Redox (reduction-oxidation) reactions include allchemical reactions in which atoms have theiroxidation 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 asoxidizing 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 asreducing 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 inoxidation number, and reduction as a decrease in oxidation number.

Although the concept ofequilibrium 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 notstatic; molecules of the substances continue to react with one another thus giving rise to adynamic equilibrium. Thus the concept describes the state in which the parameters such as chemical composition remain unchanged over time.

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

Thehistory of chemistry spans a period from the ancient past to the present. Since several millennia BC, civilizations were using technologies that would eventually form the basis of the various branches of chemistry. Examples include extractingmetals fromores, making pottery and glazes, fermenting beer and wine, extracting chemicals from plants for medicine and perfume, rendering fat intosoap, makingglass, and makingalloys likebronze.

Chemistry was preceded by its protoscience,alchemy, which operated a non-scientific approach to understanding the constituents of matter and their interactions. Despite being unsuccessful in explaining the nature of matter and its transformations, alchemists set the stage for modern chemistry by performing experiments and recording the results.Robert Boyle, although skeptical of elements and convinced of alchemy, played a key part in elevating the "sacred art" as an independent, fundamental and philosophical discipline in his workThe Sceptical Chymist (1661).^[35]

While both alchemy and chemistry are concerned with matter and its transformations, the crucial difference was given by thescientific method thatchemists employed in their work. Chemistry, as a body of knowledge distinct from alchemy, became an established science with the work ofAntoine Lavoisier, who developed a law ofconservation of mass that demanded careful measurement and quantitative observations of chemical phenomena. The history of chemistry afterwards is intertwined with thehistory of thermodynamics, especially through the work ofWillard Gibbs.^[36]

The definition of chemistry has changed over time, as new discoveries and theories add to the functionality of the science. The term "chymistry", in the view of noted scientistRobert Boyle in 1661, meant the subject of the material principles of mixed bodies.^[37] In 1663, the chemistChristopher Glaser described "chymistry" as 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 a higher perfection.^[38]

The 1730 definition of the word "chemistry", as used byGeorg Ernst Stahl, meant the art of resolving mixed, compound, or aggregate bodies into their principles; and of composing such bodies from those principles.^[39] In 1837,Jean-Baptiste Dumas considered the word "chemistry" to refer to the science concerned with the laws and effects of molecular forces.^[40] This definition further evolved until, in 1947, it came to mean the science of substances: their structure, their properties, and the reactions that change them into other substances—a characterization accepted byLinus Pauling.^[41] More recently, in 1998, ProfessorRaymond Chang broadened the definition of "chemistry" to mean the study of matter and the changes it undergoes.^[42]

Early civilizations, such as theEgyptians,^[43]Babylonians, andIndians,^[44] amassed practical knowledge concerning the arts of metallurgy, pottery and dyes, but did not develop a systematic theory.

A basic chemical hypothesis first emerged inClassical Greece with the theory offour elements as propounded definitively byAristotle stating thatfire,air,earth andwater were the fundamental elements from which everything is formed as a combination.Greekatomism dates back to 440 BC, arising in works by philosophers such asDemocritus andEpicurus. In 50 BCE, theRoman philosopherLucretius expanded upon the theory in his poemDe rerum natura (On The Nature of Things).^[45][46] Unlike modern concepts of science, Greek atomism was purely philosophical in nature, with little concern for empirical observations and no concern for chemical experiments.^[47]

An early form of the idea ofconservation of mass is the notion that "Nothing comes from nothing" inAncient Greek philosophy, which can be found inEmpedocles (approx. 4th century BC): "For it is impossible for anything to come to be from what is not, and it cannot be brought about or heard of that what is should be utterly destroyed."^[48] andEpicurus (3rd century BC), who, describing the nature of the Universe, wrote that "the totality of things was always such as it is now, and always will be".^[49]

In theHellenistic world the art of alchemy first proliferated, mingling magic and occultism into the study of natural substances with the ultimate goal of transmuting elements intogold and discovering the elixir of eternal life.^[50] Work, particularly the development ofdistillation, continued in the earlyByzantine period with the most famous practitioner being the 4th century Greek-EgyptianZosimos of Panopolis.^[51] Alchemy continued to be developed and practised throughout theArab world after theMuslim conquests,^[52] and from there, and from the Byzantine remnants,^[53] diffused into medieval andRenaissance Europe through Latin translations.

The Arabic works attributed toJabir ibn Hayyan introduced a systematic classification of chemical substances, and provided instructions for deriving an inorganic compound (sal ammoniac orammonium chloride) fromorganic substances (such as plants, blood, and hair) by chemical means.^[54] Some Arabic Jabirian works (e.g., the "Book of Mercy", and the "Book of Seventy") were later translated into Latin under theLatinized name "Geber",^[55] and in 13th-century Europe an anonymous writer, usually referred to aspseudo-Geber, started to produce alchemical and metallurgical writings under this name.^[56] Later influential Muslim philosophers, such asAbū al-Rayhān al-Bīrūnī^[57] andAvicenna^[58] disputed the theories of alchemy, particularly the theory of thetransmutation of metals.

Improvements of the refining of ores and their extractions to smelt metals was widely used source of information for early chemists in the 16th century, among themGeorg Agricola (1494–1555), who published his major workDe re metallica in 1556. His work, describing highly developed and complex processes of mining metal ores and metal extraction, were the pinnacle of metallurgy during that time. His approach removed all mysticism associated with the subject, creating the practical base upon which others could and would build. The work describes the many kinds of furnace used to smelt ore, and stimulated interest in minerals and their composition. Agricola has been described as the "father of metallurgy" and the founder ofgeology as a scientific discipline.^[62][60][61]

Under the influence of thenew empirical methods propounded bySir Francis Bacon and others, a group of chemists atOxford,Robert Boyle,Robert Hooke andJohn Mayow began to reshape the old alchemical traditions into a scientific discipline. Boyle in particular questioned some commonly held chemical theories and argued for chemical practitioners to be more "philosophical" and less commercially focused inThe Sceptical Chemyst.^[35] He formulatedBoyle's law, rejected the classical "four elements" and proposed a mechanistic alternative of atoms andchemical reactions that could be subject to rigorous experiment.^[63]

In the following decades, many important discoveries were made, such as the nature of 'air' which was discovered to be composed of many different gases. The Scottish chemistJoseph Black and the FlemishJan Baptist van Helmont discoveredcarbon dioxide, or what Black called 'fixed air' in 1754;Henry Cavendish discoveredhydrogen and elucidated its properties andJoseph Priestley and, independently,Carl Wilhelm Scheele isolated pureoxygen. The theory ofphlogiston (a substance at the root of all combustion) was propounded by the GermanGeorg Ernst Stahl in the early 18th century and was only overturned by the end of the century by the French chemistAntoine Lavoisier, the chemical analogue of Newton in physics. Lavoisier did more than any other to establish the new science on proper theoretical footing, by elucidating the principle ofconservation of mass and developing a new system of chemical nomenclature used to this day.^[65]

English scientistJohn Dalton proposed the moderntheory of atoms; that all substances are composed of indivisible 'atoms' of matter and that different atoms have varying atomic weights.

The development of the electrochemical theory of chemical combinations occurred in the early 19th century as the result of the work of two scientists in particular,Jöns Jacob Berzelius andHumphry Davy, made possible by the prior invention of thevoltaic pile byAlessandro Volta. Davy discovered nine new elements including thealkali metals by extracting them from theiroxides with electric current.^[66]

BritishWilliam Prout first proposed ordering all the elements by their atomic weight as all atoms had a weight that was an exact multiple of the atomic weight of hydrogen.J.A.R. Newlands devised an early table of elements, which was then developed into the modernperiodic table of elements^[69] in the 1860s byDmitri Mendeleev and independently by several other scientists includingJulius Lothar Meyer.^[70][71] The inert gases, later called thenoble gases were discovered byWilliam Ramsay in collaboration withLord Rayleigh at the end of the century, thereby filling in the basic structure of the table.

Organic chemistry was developed byJustus von Liebig and others, followingFriedrich Wöhler's synthesis ofurea.^[72] Other crucial 19th century advances were; an understanding of valence bonding (Edward Frankland in 1852) and the application of thermodynamics to chemistry (J. W. Gibbs andSvante Arrhenius in the 1870s).

At the turn of the twentieth century the theoretical underpinnings of chemistry were finally understood due to a series of remarkable discoveries that succeeded in probing and discovering the very nature of the internal structure of atoms. In 1897,J.J. Thomson of theUniversity of Cambridge discovered theelectron and soon after the French scientistBecquerel as well as the couplePierre andMarie Curie investigated the phenomenon ofradioactivity. In a series of pioneering scattering experimentsErnest Rutherford at theUniversity of Manchester discovered the internal structure of the atom and the existence of the proton, classified and explained the different types of radioactivity and successfullytransmuted the first element by bombardingnitrogen withalpha particles.

His work on atomic structure was improved on by his students, the Danish physicistNiels Bohr, the EnglishmanHenry Moseley and the GermanOtto Hahn, who went on to father the emergingnuclear chemistry and discoverednuclear fission. The electronic theory ofchemical bonds andmolecular orbitals was developed by the American scientistsLinus Pauling andGilbert N. Lewis.

The year 2011 was declared by the United Nations as the International Year of Chemistry.^[73] It was an initiative of the International Union of Pure and Applied Chemistry, and of the United Nations Educational, Scientific, and Cultural Organization and involves chemical societies, academics, and institutions worldwide and relied on individual initiatives to organize local and regional activities.

In the practice of chemistry,pure chemistry is the study of the fundamental principles of chemistry, whileapplied chemistry applies that knowledge to develop technology and solve real-world problems.

Chemistry is typically divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry.^[74]

• Analytical chemistry is the analysis of material samples to gain an understanding of theirchemical composition andstructure. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdisciplines of chemistry, excluding purely theoretical chemistry.^[75]

• Biochemistry is the study of thechemicals,chemical reactions and interactions that take place at a molecular level in livingorganisms. Biochemistry is highly interdisciplinary, coveringmedicinal chemistry,neurochemistry,molecular biology,forensics,plant science andgenetics.^[77]
• Inorganic chemistry is the study of the properties and reactions of inorganic compounds, such as metals and minerals.^[78] The distinction between organic and inorganic disciplines is not absolute and there is much overlap, most importantly in the sub-discipline oforganometallic chemistry.

• Materials chemistry is the preparation, characterization, and understanding of solid state components or devices with a useful current or future function.^[81] The field is a new breadth of study in graduate programs, and it integrates elements from all classical areas of chemistry likeorganic chemistry,inorganic chemistry, andcrystallography with a focus on fundamental issues that are unique tomaterials. Primary systems of study include the chemistry of condensed phases (solids, liquids,polymers) andinterfaces between different phases.
• Neurochemistry is the study ofneurochemicals; including transmitters, peptides, proteins, lipids, sugars, and nucleic acids; their interactions, and the roles they play in forming, maintaining, and modifying the nervous system.
• Nuclear chemistry is the study of how subatomic particles come together and make nuclei. Moderntransmutation is a large component of nuclear chemistry, and thetable of nuclides is an important result and tool for this field. In addition tomedical applications, nuclear chemistry encompassesnuclear engineering which explores the topic of usingnuclear power sources for generating energy.^[82][83]

• Organic chemistry is the study of the structure, properties, composition, mechanisms, andreactions oforganic compounds. An organic compound is defined as any compound based on a carbon skeleton. Organic compounds can be classified, organized and understood in reactions by theirfunctional groups, unit atoms or molecules that show characteristic chemical properties in a compound.^[85]

• 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 includechemical thermodynamics,chemical kinetics,electrochemistry,statistical mechanics,spectroscopy, and more recently,astrochemistry. Physical chemistry has large overlap withmolecular physics. Physical chemistry involves the use ofinfinitesimal calculus in deriving equations. It is usually associated withquantum chemistry and theoretical chemistry. Physical chemistry is a distinct discipline fromchemical physics, but again, there is very strong overlap.
• Theoretical chemistry is the study of chemistry via fundamental theoretical reasoning (usually withinmathematics orphysics). In particular the application ofquantum mechanics to chemistry is calledquantum chemistry. Since the end of theSecond World War, the development of computers has allowed a systematic development ofcomputational chemistry, which is the art of developing and applyingcomputer programs for solving chemical problems. Theoretical chemistry has large overlap with (theoretical and experimental)condensed matter physics andmolecular physics.

Other subdivisions includeelectrochemistry,femtochemistry,flavor chemistry,flow chemistry,immunohistochemistry,hydrogenation chemistry,mathematical chemistry,molecular mechanics,natural product chemistry,organometallic chemistry,petrochemistry,photochemistry,physical organic chemistry,polymer chemistry,radiochemistry,sonochemistry,supramolecular chemistry,synthetic chemistry, and many others.

Interdisciplinary fields includeagrochemistry,astrochemistry (andcosmochemistry),atmospheric chemistry,chemical engineering,chemical biology,chemo-informatics,environmental chemistry,geochemistry,green chemistry,immunochemistry,marine chemistry,materials science,mechanochemistry,medicinal chemistry,molecular biology,nanotechnology,oenology,pharmacology,phytochemistry,solid-state chemistry,surface science,thermochemistry, and many others.

Thechemical industry represents an important economic activity worldwide. Theglobal top 50 chemical producers in 2013 had sales ofUS$980.5 billion with a profit margin of 10.3%.^[86]

Popular reading

• Atkins, P. W.Galileo's Finger (Oxford University Press)ISBN 0-19-860941-8
• Atkins, P. W.Atkins' Molecules (Cambridge University Press)ISBN 0-521-82397-8
• Kean, Sam.The Disappearing Spoon – and Other True Tales from the Periodic Table (Black Swan) London, England, 2010ISBN 978-0-552-77750-6
• Levi, PrimoThe Periodic Table (Penguin Books) [1975] translated from the Italian by Raymond Rosenthal (1984)ISBN 978-0-14-139944-7
• Stwertka, A.A Guide to the Elements (Oxford University Press)ISBN 0-19-515027-9
• "Dictionary of the History of Ideas". Archived fromthe original on 10 March 2008.
• "Chemistry" .Encyclopædia Britannica. Vol. 6 (11th ed.). 1911. pp. 33–76.

Introductory undergraduate textbooks

• Atkins, P.W., Overton, T., Rourke, J., Weller, M. and Armstrong, F.Shriver and Atkins Inorganic Chemistry (4th ed.) 2006 (Oxford University Press)ISBN 0-19-926463-5
• Chang, Raymond.Chemistry 6th ed. Boston, Massachusetts: James M. Smith, 1998.ISBN 0-07-115221-0
• Clayden, Jonathan; Greeves, Nick;Warren, Stuart;Wothers, Peter (2001).Organic Chemistry (1st ed.). Oxford University Press.ISBN 978-0-19-850346-0.
• Voet and Voet.Biochemistry (Wiley)ISBN 0-471-58651-X

Advanced undergraduate-level or graduate textbooks

• 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
• Smart and Moore.Solid State Chemistry: An Introduction (Chapman and Hall)ISBN 0-412-40040-5
• Stephenson, G.Mathematical Methods for Science Students (Longman)ISBN 0-582-44416-0

• General Chemistry principles, patterns and applications.