Chemistry is the scientific discipline involved with compounds
composed of atoms, i.e. elements, and molecules, i.e. combinations of
atoms: their composition, structure, properties, behavior and the
changes they undergo during a reaction with other compounds.
Chemistry addresses topics such as how atoms and molecules interact
via chemical bonds to form new chemical compounds. There are four
types of chemical bonds: covalent bonds, in which compounds share one
or more electron(s); ionic bonds, in which a compound donates one or
more electrons to another compound to produce ions: cations and
anions; hydrogen bonds; and
Van der Waals force
Van der Waals force bonds. See glossary of
In the scope of its subject, chemistry occupies an intermediate
position between physics and biology. It is sometimes called the
central science because it provides a foundation for understanding
both basic and applied scientific disciplines at a fundamental
level. Examples include plant chemistry (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 (astrophysics), how medications work (pharmacology),
and how to collect DNA evidence at a crime scene (forensics).
The history of chemistry spans a period from very old times 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 extracting metals from ores,
making pottery and glazes, fermenting beer and wine, extracting
chemicals from plants for medicine and perfume, rendering fat into
soap, making glass, and making alloys like bronze.
preceded by its protoscience, alchemy, which is an intuitive but
non-scientific approach to understanding the constituents of matter
and their interactions. It was unsuccessful in explaining the nature
of matter and its transformations, but, by performing experiments and
recording the results, alchemists set the stage for modern chemistry.
Chemistry as a body of knowledge distinct from alchemy began to emerge
when a clear differentiation was made between them by
Robert Boyle in
The Sceptical Chymist
The Sceptical Chymist (1661). While both alchemy and
chemistry are concerned with matter and its transformations, the
crucial difference was given by the scientific method that chemists
employed in their work.
Chemistry is considered to have become an
established science with the work of Antoine Lavoisier, who developed
a law of conservation of mass that demanded careful measurement and
quantitative observations of chemical phenomena. The history of
chemistry is intertwined with the history of thermodynamics,
especially through the work of Willard Gibbs.
2 Modern principles
2.1.5 Substance and mixture
2.1.6 Mole and amount of substance
2.6 Ions and salts
2.7 Acidity and basicity
2.10 Chemical laws
3.1 Of definition
3.2 Of discipline
4.3 Professional societies
5 See also
8 Further reading
9 External links
The word chemistry comes from alchemy, which referred to an earlier
set of practices that encompassed elements of chemistry, metallurgy,
philosophy, astrology, astronomy, mysticism and medicine. It is often
seen as linked to the quest to turn lead or another common starting
material into gold, though in ancient times the study encompassed
many of the questions of modern chemistry being defined as the study
of the composition of waters, movement, growth, embodying,
disembodying, drawing the spirits from bodies and bonding the spirits
within bodies by the early 4th century Greek-Egyptian alchemist
Zosimos. 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".
The modern word alchemy in turn is derived from the Arabic word
al-kīmīā (الكیمیاء). In origin, the term is borrowed from
the Greek χημία or χημεία. This may have Egyptian
origins since al-kīmīā is derived from the Greek χημία, which
is in turn derived from the word Kemet, which is the ancient name of
Egypt in Egyptian. Alternately, al-kīmīā may derive from
χημεία, meaning "cast together".
Laboratory, Institute of Biochemistry,
University of Cologne
University of Cologne in
The current model of atomic structure is the quantum mechanical
model. Traditional chemistry starts with the study of elementary
particles, atoms, molecules, 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. However glassware is not central to chemistry,
and a great deal of experimental (as well as applied/industrial)
chemistry is done without it.
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. 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 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. Most chemists specialize in one or
more sub-disciplines. Several concepts are essential for the study of
chemistry; some of them are:
Main article: Matter
In chemistry, matter is defined as anything that has rest mass and
volume (it takes up space) and is made up of particles. 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.
Main article: Atom
A diagram of an atom based on the Bohr model
The atom is the basic unit of chemistry. It consists of a dense core
called the atomic nucleus surrounded by a space hosting an electron
cloud. The nucleus is made up of positively charged protons and
uncharged neutrons (together called nucleons), while the electron
cloud consists of negatively charged electrons 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 appromixately 1,836 times that of an electron, yet the
radius of an atom is about 10,000 times that of its nucleus.
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,
Standard form of the periodic table of chemical elements. The colors
represent different categories of elements
Main article: Chemical element
A chemical element is a pure substance which is composed of a single
type of atom, characterized by its particular number of protons 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 isotopes. 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.
The standard presentation of the chemical elements is in the periodic
table, which orders elements by atomic number. The periodic table is
arranged in groups, or columns, and periods, or rows. The periodic
table is useful in identifying periodic trends.
Carbon dioxide (CO2), an example of a chemical compound
Main article: 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. The standard nomenclature of compounds is set by
International Union of Pure and Applied Chemistry
International Union of Pure and Applied Chemistry (IUPAC). Organic
compounds are named according to the organic nomenclature system.
The names for
Inorganic compounds 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. 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.
Main article: Molecule
A ball-and-stick representation of the caffeine molecule (C8H10N4O2).
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
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 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 radicals.
Most radicals are comparatively reactive, but some, such as nitric
oxide (NO) can be stable.
A 2-D skeletal model of a benzene molecule (C6H6)
The "inert" or 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
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 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
Substance and mixture
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 composition
and set of properties. A collection of substances is called a
mixture. Examples of mixtures are air and alloys.
Mole and amount of substance
Main article: Mole
The mole is a unit of measurement that denotes an amount of substance
(also called chemical amount). The mole is defined as the number of
atoms found in exactly 0.012 kilogram (or 12 grams) of carbon-12,
where the carbon-12 atoms are unbound, at rest and in their ground
state. The number of entities per mole is known as the Avogadro
constant, and is determined empirically to be approximately
Molar concentration is the amount of a
particular substance per volume of solution, and is commonly reported
Diagram showing relationships among the phases and the terms used to
describe phase changes.
Main article: 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
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.
Main article: Chemical bond
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. 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.
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 while chlorine (Cl), a non-metal, gains this electron to become
Cl−. The ions are held together due to electrostatic attraction, and
that compound sodium chloride (NaCl), or common table salt, is formed.
In the methane molecule (CH4), 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 electrons 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
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.
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
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
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 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.
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
Gibbs free energy is negative,
displaystyle Delta Gleq 0,
; if it is equal to zero the chemical reaction is said to be at
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.
Whereas hydrogen sulfide (H2S) is a gas at room temperature and
standard pressure, as its molecules are bound by weaker dipole-dipole
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
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.
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 as with the system
environment, which may be designed vessels—often laboratory
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
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 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."
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
Ions and salts
The crystal lattice structure of potassium chloride (KCl), a salt
which is formed due to the attraction of K+ cations and Cl− anions.
Note how the overall charge of the ionic compound is zero.
Main article: 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 salts, such as
the Na+ and Cl− ions forming sodium chloride, or NaCl. Examples of
polyatomic ions that do not split up during acid-base reactions are
hydroxide (OH−) and phosphate (PO43−).
Plasma is composed of gaseous matter that has been completely ionized,
usually through high temperature.
Acidity and basicity
When hydrogen bromide (HBr), pictured, is dissolved in water, it forms
the strong acid hydrobromic acid
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 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
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. 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
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 Brønsted–Lowry
definition, is the acid dissociation constant (Ka), 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 Ka are more likely to donate hydrogen ions in chemical
reactions than those with lower Ka values.
Main article: Redox
Redox (reduction-oxidation) reactions include all chemical reactions
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.
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, 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; 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.
Main article: Chemical law
Chemical reactions are governed by certain laws, which have become
fundamental concepts in chemistry. Some of them are:
Boyle's law (1662, relating pressure and volume)
Charles's law (1787, relating volume and temperature)
Fick's laws of diffusion
Gay-Lussac's law (1809, relating pressure and temperature)
Le Chatelier's principle
Law of conservation of energy leads to the important concepts of
equilibrium, thermodynamics, and kinetics.
Law of conservation of mass
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
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 scientist
Robert Boyle in 1661,
meant the subject of the material principles of mixed bodies. In
1663 the chemist
Christopher 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.
The 1730 definition of the word "chemistry", as used by Georg Ernst
Stahl, meant the art of resolving mixed, compound, or aggregate bodies
into their principles; and of composing such bodies from those
principles. In 1837,
Jean-Baptiste Dumas considered the word
"chemistry" to refer to the science concerned with the laws and
effects of molecular forces. 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 by Linus Pauling.
More recently, in 1998, Professor Raymond Chang broadened the
definition of "chemistry" to mean the study of matter and the changes
Main article: History of chemistry
Alchemy and Timeline of chemistry
Democritus' atomist philosophy was later adopted by Epicurus
Early civilizations, such as the Egyptians Babylonians,
Indians amassed practical knowledge concerning the arts of
metallurgy, pottery and dyes, but didn't develop a systematic theory.
A basic chemical hypothesis first emerged in
Classical Greece with the
theory of four elements as propounded definitively by Aristotle
stating that fire, air, earth and water were the fundamental elements
from which everything is formed as a combination. Greek atomism dates
back to 440 BC, arising in works by philosophers such as Democritus
and Epicurus. In 50 BC, the Roman philosopher
Lucretius expanded upon
the theory in his book
De rerum natura
De rerum natura (On The Nature of
Things). Unlike modern concepts of science, Greek atomism was
purely philosophical in nature, with little concern for empirical
observations and no concern for chemical experiments.
Hellenistic world the art of alchemy first proliferated,
mingling magic and occultism into the study of natural substances with
the ultimate goal of transmuting elements into gold and discovering
the elixir of eternal life. Work, particularly the development of
distillation, continued in the early
Byzantine period with the most
famous practitioner being the 4th century Greek-Egyptian Zosimos of
Alchemy continued to be developed and practised
Arab world after the Muslim conquests, and from
there, and from the
Byzantine remnants, diffused into medieval and
Renaissance Europe through Latin translations. Some influential Muslim
chemists, Abū al-Rayhān al-Bīrūnī, Avicenna and Al-Kindi
refuted the theories of alchemy, particularly the theory of the
transmutation of metals; and al-Tusi described a version of the
conservation of mass, noting that a body of matter is able to change
but is not able to disappear.
Jābir ibn Hayyān
Jābir ibn Hayyān (Geber), a Perso-Arab alchemist whose experimental
research laid the foundations of chemistry.
The development of the modern scientific method was slow and arduous,
but an early scientific method for chemistry began emerging among
early Muslim chemists, beginning with the 9th century Perso-Arab
Jābir ibn Hayyān
Jābir ibn Hayyān (known as "Geber" in Europe), who is
sometimes referred to as "the father of chemistry". He
introduced a systematic and experimental approach to scientific
research based in the laboratory, in contrast to the ancient Greek and
Egyptian alchemists whose works were largely allegorical and often
unintelligble. Under the influence of the new empirical methods
Sir Francis Bacon
Sir Francis Bacon and others, a group of chemists at
Oxford, Robert Boyle,
Robert Hooke and
John Mayow began to reshape the
old alchemical traditions into a scientific discipline. Boyle in
particular is regarded as the founding father of chemistry due to his
most important work, the classic chemistry text The Sceptical Chymist
where the differentiation is made between the claims of alchemy and
the empirical scientific discoveries of the new chemistry. He
formulated Boyle's law, rejected the classical "four elements" and
proposed a mechanistic alternative of atoms and chemical reactions
that could be subject to rigorous experiment.
Antoine-Laurent de Lavoisier
Antoine-Laurent de Lavoisier is considered the "Father of Modern
The theory of phlogiston (a substance at the root of all combustion)
was propounded by the German
Georg Ernst Stahl
Georg Ernst Stahl in the early 18th
century and was only overturned by the end of the century by the
French chemist Antoine Lavoisier, the chemical analogue of Newton in
physics; who did more than any other to establish the new science on
proper theoretical footing, by elucidating the principle of
conservation of mass and developing a new system of chemical
nomenclature used to this day.
Before his work, though, many important discoveries had been made,
specifically relating to the nature of 'air' which was discovered to
be composed of many different gases. The Scottish chemist Joseph Black
(the first experimental chemist) and the Dutchman J. B. van Helmont
discovered carbon dioxide, or what Black called 'fixed air' in 1754;
Henry Cavendish discovered hydrogen and elucidated its properties and
Joseph Priestley and, independently,
Carl Wilhelm Scheele
Carl Wilhelm Scheele isolated
In his periodic table,
Dmitri Mendeleev predicted the existence of 7
new elements, and placed all 60 elements known at the time in
their correct places.
John Dalton proposed the modern theory 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,
J. J. Berzelius
J. J. Berzelius and Humphry Davy, made
possible by the prior invention of the voltaic pile by Alessandro
Volta. Davy discovered nine new elements including the alkali metals
by extracting them from their oxides with electric current.
William 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
J. A. R. Newlands devised
an early table of elements, which was then developed into the modern
periodic table of elements in the 1860s by
Dmitri Mendeleev and
independently by several other scientists including Julius Lothar
Meyer. The inert gases, later called the noble gases were
William Ramsay in collaboration with
Lord Rayleigh at
the end of the century, thereby filling in the basic structure of the
Top: Expected results: alpha particles passing through the plum
pudding model of the atom undisturbed.
Bottom: Observed results: a small portion of the particles were
deflected, indicating a small, concentrated charge.
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
J. J. Thomson of
Cambridge University discovered the electron and soon after the French
Becquerel as well as the couple Pierre and Marie Curie
investigated the phenomenon of radioactivity. In a series of
pioneering scattering experiments
Ernest Rutherford at the University
of Manchester discovered the internal structure of the atom and the
existence of the proton, classified and explained the different types
of radioactivity and successfully transmuted the first element by
bombarding nitrogen with alpha particles.
His work on atomic structure was improved on by his students, the
Niels Bohr and Henry Moseley. The electronic theory
of chemical bonds and molecular orbitals was developed by the American
Linus Pauling and Gilbert N. Lewis.
The year 2011 was declared by the United Nations as the International
Year of Chemistry. 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.
Organic chemistry was developed by
Justus von Liebig
Justus von Liebig and others,
following Friedrich Wöhler's synthesis of urea which proved that
living organisms were, in theory, reducible to chemistry. Other
crucial 19th century advances were; an understanding of valence
Edward Frankland in 1852) and the application of
thermodynamics to chemistry (
J. W. Gibbs
J. W. Gibbs and
Svante Arrhenius in the
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article by introducing citations to additional sources. (September
Chemistry is typically divided into several major sub-disciplines.
There are also several main cross-disciplinary and more specialized
fields of chemistry.
Analytical chemistry is the analysis of material samples to gain an
understanding of their chemical composition and structure. Analytical
chemistry incorporates standardized experimental methods in chemistry.
These methods may be used in all 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
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. 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
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.
Main article: Chemical industry
The chemical industry represents an important economic activity
worldwide. The global top 50 chemical producers in 2013 had sales of
US$980.5 billion with a profit margin of 10.3%.
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
Outline of chemistry
Glossary of chemistry terms
International Year of Chemistry
List of chemists
List of compounds
List of important publications in chemistry
Comparison of software for molecular mechanics modeling
List of unsolved problems in chemistry
Periodic systems of small molecules
Philosophy of chemistry
^ "What is Chemistry?". Chemweb.ucc.ie. Retrieved 2011-06-12.
^ Chemistry. (n.d.). Merriam-Webster's Medical Dictionary. Retrieved
August 19, 2007.
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Boundaries. Wiley-VCH, 2001. ISBN 3-527-30271-9. pp. 1–2.
^ Theodore L. Brown, H. Eugene Lemay, Bruce Edward Bursten, H. Lemay.
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ISBN 0-13-010310-1. pp. 3–4.
^ Selected Classic Papers from the History of Chemistry
^ "History of Alchemy".
Alchemy Lab. Retrieved 2011-06-12.
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Elements. New York: Berkley Books.
^ a b "alchemy", entry in The
Oxford English Dictionary, J. A. Simpson
and E. S. C. Weiner, vol. 1, 2nd ed., 1989, ISBN 0-19-861213-3.
^ p. 854, "Arabic alchemy", Georges C. Anawati, pp. 853–885 in
Encyclopedia of the history of Arabic science, eds. Roshdi Rashed and
Régis Morelon, London: Routledge, 1996, vol. 3,
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T. Nomenclature of
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Chemistry (4th ed.). Upper Saddle River, New Jersey: Pearson
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^ M. M. Avedesian; Hugh Baker. Magnesium and Magnesium Alloys. ASM
International. p. 59.
^ "Official SI Unit definitions". Bipm.org. Retrieved
^ Burrows et al. 2008, p. 16.
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Matter - Chemforkids.com
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^ "History of Acidity". Bbc.co.uk. 2004-05-27. Retrieved
^ Boyle, Robert (1661). The Sceptical Chymist. New York: Dover
Publications, Inc. (reprint). ISBN 0-486-42825-7.
^ Glaser, Christopher (1663). Traite de la chymie. Paris. as
found in: Kim, Mi Gyung (2003). Affinity, That Elusive Dream – A
Genealogy of the Chemical Revolution. The MIT Press.
^ Stahl, George, E. (1730). Philosophical Principles of Universal
^ Dumas, J. B. (1837). 'Affinite' (lecture notes), vii, pg 4.
"Statique chimique", Paris: Académie des Sciences
^ Pauling, Linus (1947). General Chemistry. Dover Publications, Inc.
^ Chang, Raymond (1998). Chemistry, 6th Ed. New York: McGraw Hill.
^ First chemists, February 13, 1999, New Scientist
^ Barnes, Ruth. Textiles in Indian Ocean Societies. Routledge.
Lucretius (50 BCE). "de Rerum Natura (On the Nature of Things)". The
Internet Classics Archive. Massachusetts Institute of Technology.
Retrieved 9 January 2007. Check date values in: date= (help)
^ Simpson, David (29 June 2005). "
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The Internet History of Philosophy. Retrieved 2007-01-09.
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Classics. pp. 7–8. ISBN 0-14-310721-6.
International Year of Chemistry
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Science and Technology. Houghton Mifflin Harcourt. p. 88.
Morris Kline (1985)
Mathematics for the nonmathematician. Courier
Dover Publications. p. 284. ISBN 0-486-24823-2
^ Marcelin Berthelot, Collection des anciens alchimistes grecs (3
vol., Paris, 1887–1888, p. 161); F. Sherwood Taylor, "The Origins of
Greek Alchemy," Ambix 1 (1937), 40.
^ Marmura, Michael E.; Nasr, Seyyed Hossein (1965). "An Introduction
to Islamic Cosmological Doctrines. Conceptions of Nature and Methods
Used for Its Study by the Ikhwan Al-Safa'an, Al-Biruni, and Ibn Sina
by Seyyed Hossein Nasr". Speculum. 40 (4): 744–746.
doi:10.2307/2851429. JSTOR 2851429.
Robert Briffault (1938). The Making of Humanity, pp. 196–197.
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crystallography". Acta Crystallographica Section A. 64 (Pt 1):
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^ John Warren (2005). "War and the Cultural Heritage of Iraq: a sadly
mismanaged affair", Third World Quarterly,
Volume 26, Issue 4 & 5,
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^ Paul Vallely, How Islamic inventors changed the world, The
Independent, 10 March 2006
^ Kraus, Paul, Jâbir ibn Hayyân, Contribution à l'histoire des
idées scientifiques dans l'Islam. I. Le corpus des écrits
jâbiriens. II. Jâbir et la science grecque,. Cairo (1942–1943).
Repr. By Fuat Sezgin, (Natural Sciences in Islam. 67–68), Frankfurt.
"To form an idea of the historical place of Jabir's alchemy and to
tackle the problem of its sources, it is advisable to compare it with
what remains to us of the alchemical literature in the Greek language.
One knows in which miserable state this literature reached us.
Byzantine scientists from the tenth century, the corpus
of the Greek alchemists is a cluster of incoherent fragments, going
back to all the times since the third century until the end of the
"The efforts of Berthelot and Ruelle to put a little order in this
mass of literature led only to poor results, and the later
researchers, among them in particular Mrs. Hammer-Jensen, Tannery,
Lagercrantz, von Lippmann, Reitzenstein, Ruska, Bidez, Festugiere and
others, could make clear only few points of detail…
The study of the Greek alchemists is not very encouraging. An even
surface examination of the Greek texts shows that a very small part
only was organized according to true experiments of laboratory: even
the supposedly technical writings, in the state where we find them
today, are unintelligible nonsense which refuses any interpretation.
It is different with Jabir's alchemy. The relatively clear description
of the processes and the alchemical apparatuses, the methodical
classification of the substances, mark an experimental spirit which is
extremely far away from the weird and odd esotericism of the Greek
texts. The theory on which Jabir supports his operations is one of
clearness and of an impressive unity. More than with the other Arab
authors, one notes with him a balance between theoretical teaching and
practical teaching, between the `ilm and the `amal. In vain one would
seek in the Greek texts a work as systematic as that which is
presented for example in the
Book of Seventy."
(cf. Ahmad Y Hassan. "A Critical Reassessment of the Geber Problem:
Part Three". Archived from the original on 2008-11-20. Retrieved
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^ "History –
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the Chemical Revolution. MIT Press. p. 440.
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Julius Lothar Meyer
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^ "What makes these family likenesses among the elements? In the 1860s
everyone was scratching their heads about that, and several scientists
moved towards rather similar answers. The man who solved the problem
most triumphantly was a young Russian called Dmitri Ivanovich
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Jacob (1973). The Ascent of Man. Little, Brown and Company.
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Atkins, Peter; de Paula, Julio (2009) . Elements of Physical
Chemistry (5th ed.). New York:
Oxford University Press.
Burrows, Andrew; Holman, John; Parsons, Andrew; Pilling, Gwen; Price,
Gareth (2009). Chemistry3. Italy:
Oxford University Press.
Housecroft, Catherine E.; Sharpe, Alan G. (2008) . Inorganic
Chemistry (3rd ed.). Harlow, Essex: Pearson Education.
Atkins, P.W. Galileo's Finger (
Oxford University Press)
Atkins, P.W. Atkins'
Cambridge University Press)
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)  translated from
the Italian by Raymond Rosenthal (1984) ISBN 978-0-14-139944-7
Stwertka, A. A Guide to the Elements (
Oxford University Press)
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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
Chemistry 6th ed. Boston: James M. Smith, 1998.
Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter
Chemistry (1st ed.).
Oxford University Press.
Voet and Voet
Biochemistry (Wiley) ISBN 0-471-58651-X
Advanced undergraduate-level or graduate text books
Atkins, P.W. Physical
Oxford University Press)
Atkins, P.W. et al. Molecular
Quantum Mechanics (
McWeeny, R. Coulson's Valence (
Oxford Science Publications)
Pauling, L. The Nature of the chemical bond (Cornell University Press)
Pauling, L., and Wilson, E. B. Introduction to
Quantum Mechanics with
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)
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