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Chemicals in flasks (including Ammonium hydroxide and Nitric acid) lit in different colors

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Chemistry, a branch of physical science, is the study of the composition, properties and behavior of matter.[1][2] Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds. Chemistry is also concerned with the interactions between atoms (or groups of atoms) and various forms of energy (e.g. photochemical reactions, changes in phases of matter, separation of mixtures, properties of polymers, etc.).

Chemistry is sometimes called "the central science" because it connects physics with other natural sciences such as geology and biology.[3][4] Chemistry is a branch of physical science but distinct from physics.[5]

The etymology of the word chemistry has been much disputed.[6] 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.[7]

## Etymology[edit]

Main article: Chemistry (etymology)

The word chemistry comes from the word alchemy, an earlier set of practices that encompassed elements of chemistry, metallurgy, philosophy, astrology, astronomy, mysticism and medicine; it is commonly thought of as the quest to turn lead or another common starting material into gold.[8] The word alchemy in turn is derived from the Arabic word al-k?m?? (????????). The Arabic term is borrowed from the Greek ????? or ??????.[9][10] This may have Egyptian origins. Many believe that al-k?m?? is derived from ?????, which is in turn derived from the word Chemi or Kimi, which is the ancient name of Egypt in Egyptian.[9] Alternately, al-k?m?? may be derived from ??????, meaning "cast together".[11]

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 has changed over time, 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, disembodying, drawing the spirits from bodies and bonding the spirits within bodies (Zosimos).[12]
- Chymistry (1661) ? the subject of the material principles of mixed bodies (Boyle).[13]
- 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 a higher perfection (Glaser).[14]
- Chemistry (1730) ? the art of resolving mixed, compound, or aggregate bodies into their principles; and of composing such bodies from those principles (Stahl).[15]
- Chemistry (1837) ? the science concerned with the laws and effects of molecular forces (Dumas).[16]
- Chemistry (1947) ? the science of substances: their structure, their properties, and the reactions that change them into other substances (Pauling).[17]
- Chemistry (1998) ? the study of matter and the changes it undergoes (Chang).[18]

## History[edit]

Main article: History of chemistry
See also: Alchemy, Timeline of chemistry, and Nobel Prize in Chemistry

Ancient Egyptians pioneered the art of synthetic "wet" chemistry up to 4,000 years ago.[19] By 1000 BC ancient civilizations were using technologies that formed the basis of the various branches of chemistry such as; extracting metal from their ores, making pottery and glazes, fermenting beer and wine, making pigments for cosmetics and painting, extracting chemicals from plants for medicine and perfume, making cheese, dying cloth, tanning leather, rendering fat into soap, making glass, and making alloys like bronze.

Democritus' atomist philosophy was later adopted by Epicurus (341?270 BCE).

The genesis of chemistry can be traced to the widely observed phenomenon of burning that led to metallurgy?the art and science of processing ores to get metals (e.g. 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.[20]

Greek atomism dates back to 440 BC, as what might be indicated by the book De Rerum Natura (The Nature of Things)[21] written by the Roman Lucretius in 50 BC.[22] Much of the early development of purification methods is described by Pliny the Elder in his Naturalis Historia.

A tentative outline is as follows:

1. Alchemy in Greco-Roman Egypt [ ? 642 CE], the earliest Western alchemists such as Mary the Jewess, Cleopatra the Alchemist, and Zosimos of Panopolis described early laboratory equipment. They are estimated to have lived between the first and third centuries.
2. Islamic alchemy [642 CE ? 1200], the Muslim conquest of Egypt; development of alchemy by J?bir ibn Hayy?n, al-Razi and others; J?bir modifies Aristotle's theories; advances in processes and apparatus.[23]
3. European alchemy [1300 ? present], Pseudo-Geber builds on Arabic chemistry.[citation needed] From the 12th century, major advances in the chemical arts shifted from Arab lands to western Europe.[23]
4. Chemistry [1661], Boyle writes his classic chemistry text The Sceptical Chymist.
5. Chemistry [1787], Lavoisier writes his classic Elements of Chemistry.
6. Chemistry [1803], Dalton publishes his Atomic Theory.
7. Chemistry [1869], Dmitri Mendeleev presented his Periodic table being the framework of the modern chemistry

The earliest pioneers of chemistry and the scientific method,[24] were medieval Arab and Persian scholars. They introduced precise observation and controlled experimentation into the field and discovered numerous chemical substances.[25][verification needed]

> "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 Saracens introduced precise observation, controlled experiment, and careful records. They invented and named the alembic (al-anbiq), chemically analyzed innumerable substances, composed lapidaries, distinguished alkalis and acids, investigated their affinities, studied and manufactured hundreds of drugs. 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."[25]

The most influential Muslim chemists were J?bir ibn Hayy?n (Geber, d. 815), al-Kindi (d. 873), al-Razi (d. 925), al-Biruni (d. 1048) and Alhazen (d. 1039).[26] Their works became more widely known in Europe in the twelfth and thirteenth centuries, beginning with the Latin translation of J?bir?s Kitab al-Kimya in 1144. The contribution of Indian alchemists and metallurgists in the development of chemistry was also quite significant.[27]

Antoine-Laurent de Lavoisier is considered the "Father of Modern Chemistry".[28]

For some practitioners alchemy was an intellectual pursuit, and 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 Ren? 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 equation known as Boyle's Law about the characteristics of gaseous state.[29]

Chemistry 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[citation needed] 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[citation needed] to fit all experiments into the framework of a single theory.

Lavoisier 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 contribution 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 contributions that Antoine Lavoisier is often celebrated as the "Father of Modern Chemistry".[30] The later discovery of Friedrich W?hler that many natural substances, organic compounds, can indeed be synthesized in a chemistry laboratory also helped the modern chemistry to mature from its infancy.[31]

The discovery 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)[32] and later discoveries of some synthetic elements.

J?ns Jacob Berzelius, Joseph Priestley, Humphry Davy, Linus Pauling, Gilbert N. Lewis, Josiah Willard Gibbs, Robert Burns Woodward, and Fritz Haber also made notable contributions.

The year 2011 was declared by the United Nations as the International Year of Chemistry.[33] 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.

## Principles of modern chemistry[edit]

Laboratory, Institute of Biochemistry, University of Cologne.

The current model of atomic structure is the quantum mechanical model.[34] Traditional chemistry starts with the study of elementary particles, atoms, molecules,[35] substances, metals, crystals and other aggregates of matter. This matter can be studied in solid, liquid, or gas states, 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 the chemical bonds which hold atoms together. Such behaviors are studied in a chemistry laboratory.

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

A chemical reaction is a transformation of some substances into one or more different substances.[36] 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 unequal, the transformation by definition is not chemical, but rather 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 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 compositions. They can be analyzed using the tools of chemical analysis, e.g. spectroscopy and chromatography. Scientists engaged in chemical research are known as chemists.[37] Most chemists specialize in one or more sub-disciplines. Several concepts are essential for the study of chemistry; some of them are:[38]

### Matter[edit]

#### Atom[edit]

Main article: Atom
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 called the electron cloud. The nucleus is made up of positively charged protons and neutral neutrons, while the electron cloud consists of negatively-charged electrons which orbit the nucleus to balance out the positive charge of the protons.

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., metallic, ionic, covalent).

#### Element[edit]

Main article: Chemical element

The concept of chemical element is related to that of chemical substance. A chemical element is specifically a substance which is composed of a single type of atom. A chemical element is characterized by a particular number of protons in the 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.

Although all the nuclei of all atoms belonging to one element will have the same number of protons, they may not necessarily have the same number of neutrons; such atoms are termed isotopes. In fact several isotopes of an element may exist. Ninety?four different chemical elements or types of atoms based on the number of protons are observed on earth naturally, having at least one isotope that is stable or has a very long half-life. A further 18 elements have been recognised by IUPAC after they have been made in the laboratory.

The standard presentation of the chemical elements is in the periodic table, which orders elements by atomic number and groups them by electron configuration. Due to its arrangement, 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 by name, by symbol, and by atomic number are also available.

#### Compound[edit]

Main article: Chemical compound

A compound is a substance with a particular ratio of atoms of particular chemical elements which determines its composition, and a particular organization which determines its chemical properties. For example, water is a compound containing hydrogen and oxygen in the ratio of two to one, with the oxygen atom between the two hydrogen atoms, and an angle of 104.5? between them. Compounds are formed and interconverted by chemical reactions.

#### Substance[edit]

Main article: Chemical substance

A chemical substance is a kind of matter with a definite composition and set of properties.[39] 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; for example: air, alloys, biomass, etc.

Nomenclature of substances is a critical part of the language of chemistry. Generally it refers to a system for naming chemical compounds. 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 vast 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 compounds are named according to the organic nomenclature system.[40] Inorganic compounds are named according to the inorganic nomenclature system.[41] In addition the Chemical Abstracts Service 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[edit]

Main article: Molecule

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 bonds, such that the structure is electrically neutral and all valence electrons are paired with other electrons either in bonds or in lone 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 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 spectrograph. Charged polyatomic collections residing in solids (for example, common sulfate or nitrate ions) are generally not considered "molecules" in chemistry.

A molecular structure depicts the bonds and relative positions of atoms in a molecule such as that in Paclitaxel shown here

The "inert" or noble chemical 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 pharmaceuticals.

However, not all substances or chemical compounds consist of discrete molecules, and indeed most of the solid substances that makes 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 units or unit cells 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 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 and amount of substance[edit]

Main article: Mole (unit)

The mole is a unit of measurement that denotes an amount of substance (also called chemical amount). Specifically it is the amount of a substance that contains as many elementary entities (atoms, molecules or ions) as there are atoms in 0.012 kilogram (or 12?grams) of carbon-12, where the carbon-12 atoms are unbound, at rest and in their ground state.[42] The number of entities per mole is known as the Avogadro constant, and is determined empirically. The currently accepted value is 6.02214179(30)×1023 mol?1 (2007 CODATA). One way to understand the meaning of the term "mole" is to compare and contrast it to terms such as dozen. Just as one dozen eggs contains 12 individual eggs, one mole contains 6.02214179(30)×1023 atoms, molecules or other particles. The term is used because it is much easier to say, for example, 1 mole of carbon, than it is to say 6.02214179(30)×1023 carbon atoms, and because moles of chemicals represent a scale that is easy to experience.

The amount of substance of a solute per volume of solution is known as amount of substance concentration, or molarity for short. Molarity is the quantity most commonly used to express the concentration of a solution in the chemical laboratory. The most commonly used units for molarity are mol/L (the official SI units are mol/m3).

### Properties[edit]

#### Ions and salts[edit]

Main article: 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 ) and negatively charged anions (e.g. chloride Cl?) can form a crystalline lattice of neutral salts (e.g. sodium chloride NaCl). Examples of polyatomic ions that do not split up during acid-base reactions are hydroxide (OH?) and phosphate (PO43?).

Ions in the gaseous phase are often known as plasma.

#### Acidity and basicity[edit]

Main article: Acid?base reaction

A substance can often be classified as an acid or a base. There are several different theories which explain acid-base behavior. The simplest is Arrhenius theory, which states than an acid is a substance that produces hydronium ions when it is dissolved in water, and a base is one that produces hydroxide ions when dissolved in water. According to Br?nsted?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 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. According to this theory, the crucial things being exchanged are charges.[43][unreliable source?] 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 [44]

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 logarithmic 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 Br?nsted?Lowry definition, is the acid dissociation constant (Ka), which measure the relative ability of a substance to act as an acid under the Br?nsted?Lowry definition of an acid. That is, substances with a higher Ka are more likely to donate hydrogen ions in chemical reactions than those with lower Ka values.

#### Phase[edit]

Main article: 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 state. When three states meet based on the conditions, it is known as a triple point and since this is invariant, it is a convenient way to define a set of conditions.

The most familiar examples of phases are solids, liquids, and gases. 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 plasmas, Bose-Einstein condensates and fermionic condensates and the paramagnetic and ferromagnetic phases of magnetic 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[edit]

Main article: Chemical bond
Electron atomic and molecular orbitals

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.[45] More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom.

A 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 molecules or crystals. 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.

Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes, valence bond theory is less applicable and alternative approaches, such as the molecular orbital theory, are generally used. See diagram on electronic orbitals.

### Reaction[edit]

Main article: Chemical reaction
During chemical reactions, bonds between atoms break and form, resulting in different substances with different properties. In a blast furnace, iron oxide, a compound, reacts with carbon monoxide to form iron, one of the chemical elements, 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; exposure to some form of energy, or both. It results in some energy exchange between the constituents of the reaction as well with the system environment which may be designed vessels which are often laboratory glassware.

Chemical reactions can result in the formation or dissociation of molecules, that is, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds. Oxidation, reduction, 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.[46]

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 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 kinetics and the relative product mix of a reaction. Many 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.

According to the IUPAC gold book a chemical reaction is a process that results in the interconversion of chemical species".[47] 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 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').

#### Redox[edit]

Main article: Redox

Redox(reduction-oxidation) reactions include all chemical reactions in which atoms have their oxidation state changed by either loseing electrons(reduction) or gaining 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[edit]

Main article: Chemical equilibrium

Although the concept of 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. 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; 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.

#### Energy[edit]

Main article: Energy

In the context of chemistry, energy is an attribute of a substance as a consequence of its atomic, molecular or aggregate 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 increase or 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 light; thus the products of a reaction may have more or less energy than the reactants.

A reaction is said to be exergonic if the final state is lower on the energy scale than the initial state; in the case of endergonic reactions the situation is the reverse. A reaction is said to be exothermic if the reaction releases heat to the surroundings; in the case of endothermic reactions, 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 - 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.[48]

A related concept 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, ; if it is equal to zero the chemical reaction is said to be at 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 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 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 by hydrogen bonds.[49] Whereas hydrogen sulfide (H2S) is a gas at room temperature and standard pressure, as its molecules are bound by weaker dipole-dipole interactions.

The transfer of energy from one chemical substance to another depends on the size of energy 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 substances is useful for their identification by the analysis of spectral lines. Different kinds of spectra are often used in chemical spectroscopy, 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.

Emission spectrum of iron

The term 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.

### Chemical laws[edit]

Main article: Chemical law

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

- Avogadro's law
- Beer-Lambert law
- Boyle's law (1662, relating pressure and volume)
- Charles's law (1787, relating volume and temperature)
- Fick's law of diffusion
- Gay-Lussac's law (1809, relating pressure and temperature)
- Le Chatelier's Principle
- Henry's law
- Hess's Law
- Law of conservation of energy leads to the important concepts of equilibrium, thermodynamics, and kinetics.
- Law of conservation of mass continues to be conserved in isolated systems, 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

## Practice[edit]

### Subdisciplines[edit]

Chemistry is typically divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry.[50]

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

- Biochemistry is the study of the chemicals, chemical reactions and chemical interactions that take place in living organisms. Biochemistry and organic chemistry are closely related, 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 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.

- Neurochemistry is the study of neurochemicals; 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. Modern 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 reactions of organic compounds. 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, spectroscopy, and more recently, astrochemistry.[51] Physical chemistry has large overlap with molecular physics. Physical chemistry involves the use of infinitesimal calculus in deriving equations. It is usually associated with quantum chemistry and theoretical chemistry. Physical chemistry is a distinct discipline from chemical physics, but again, there is very strong overlap.

- 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 Second World War, the development of computers has allowed a systematic development of computational chemistry, which is the art of developing and applying computer programs for solving chemical problems. Theoretical chemistry has large overlap with (theoretical and experimental) condensed matter physics and molecular physics.

Other 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 (carbon based) matter; biochemistry, the study of substances found in biological organisms; physical chemistry, the study of chemical processes using physical concepts such as thermodynamics and quantum mechanics; and analytical chemistry, the analysis of material samples to gain an understanding of their chemical composition and structure. Many more specialized disciplines have emerged in recent years, e.g. neurochemistry the chemical study of the nervous system (see subdisciplines).

Other fields include agrochemistry, astrochemistry (and cosmochemistry), atmospheric chemistry, chemical engineering, chemical biology, chemo-informatics, electrochemistry, environmental chemistry, femtochemistry, flavor chemistry, flow chemistry, geochemistry, green chemistry, histochemistry, history of chemistry, hydrogenation chemistry, immunochemistry, marine chemistry, materials science, mathematical chemistry, mechanochemistry, medicinal chemistry, molecular biology, molecular mechanics, nanotechnology, natural product chemistry, oenology, organometallic chemistry, petrochemistry, pharmacology, photochemistry, physical organic chemistry, phytochemistry, polymer chemistry, radiochemistry, solid-state chemistry, sonochemistry, supramolecular chemistry, surface chemistry, synthetic chemistry, thermochemistry, and many others.

### Chemical industry[edit]

Main article: 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.[52]

### Professional societies[edit]

- American Chemical Society
- American Society for Neurochemistry
- Chemical Institute of Canada
- Chemical Society of Peru
- International Union of Pure and Applied Chemistry
- Royal Australian Chemical Institute
- Royal Netherlands Chemical Society
- Royal Society of Chemistry
- Society of Chemical Industry
- World Association of Theoretical and Computational Chemists
- List of chemistry societies

## See also[edit]

| Book: Chemistry

Main article: Outline of chemistry

- Glossary of chemistry terms
- Common chemicals
- International Year of Chemistry
- List of chemists
- List of compounds
- List of important publications in chemistry
- List of software for molecular mechanics modeling
- List of unsolved problems in chemistry
- Periodic Systems of Small Molecules
- Periodic table
- Philosophy of chemistry

## References[edit]

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    > "Humboldt regards the Muslims as the founders of chemistry."

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## Further reading[edit]

Find more about Chemistry at Wikipedia's sister projects
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| Media from Commons
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| Quotations from Wikiquote
| Source texts from Wikisource
| Textbooks from Wikibooks

| Wikiversity has learning materials about chemistry at The School of Chemistry

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, 2010 ISBN 978-0-552-77750-6
- Levi, Primo The 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

Introductory undergraduate text books

- 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
- Chang, Raymond. Chemistry 6th ed. Boston: 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 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
- 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

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