Chemical compound

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Chemical compound

A chemical compound is a substance

Chemical substance

A chemical substance is a material with a specific Empirical formula. It is a concept that became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate....
consisting of two or more different elements

Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its atomic nucleus. The term is also used to refer to a pure chemical Chemical substance composed of atoms with the same number of protons....
bonded

Chemical bond

A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds....
together in a fixed mass

Mass

In physical science, mass refers to the degree of acceleration a body acquires when subject to a force: bodies with greater mass are accelerated less by the same force....
ratio that can be split into simpler substances.
Fine-tuning the definition
There are some exceptions to the definition above. Certain crystalline

Crystal structure

In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice....
compounds are non-stoichiometric compound

Non-stoichiometric compound

Non-stoichiometric compounds are chemical compounds with an chemical element composition that cannot be represented by a ratio of well-defined natural numbers, and therefore violate the law of definite proportions....
s because they vary in composition according to the presence or otherwise of elements trapped within the crystal structure.

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Fine-tuning the definition

There are some exceptions to the definition above. Certain crystalline

Crystal structure

Non-stoichiometric compound

Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
s of the constituent elements, which will make the ratio of elements by mass vary slightly. A compound therefore may not be completely homogenous, but for most purposes in chemistry it can be regarded as such.

Compounds compared to mixtures

Compounds have different physical

Physical property

A physical property is any aspect of an object or substance that can be measurement or perception without changing its Identity . Physical properties can be Intensive and extensive properties....
and chemical properties

Chemical property

A chemical property is any of a material's properties that becomes evident during a chemical reaction; that is, any quality that can be established only by changing a substance's Chemical substance....
from their constituent elements. This is the one principal criterion for distinguishing a compound from a mixture

Mixture

In chemistry, a mixture is a substance made by combining two or more different materials without a chemical reaction occurring .While there are no physical changes in a mixture, the chemical properties of a mixture, such as its melting point, may differ from those of its components....
of elements or substances: a mixture's properties are generally similar or related to the properties of its constituents. Another criterion is that the constituents of a mixture can usually be separated by simple, mechanical means such as filtering, those of a compound are often very hard to separate. Furthermore, when a compound is formed from its constituents, a chemical change

Chemical change

In a chemical reaction, chemical bonds are broken and new bonds are formed between different atoms. This breaking and forming of bonds takes place when particles of the original materials collide with one another....
takes place through chemical reaction

Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
s. Mixtures can be made by mechanical means alone.

An example of a mixture that is often confused to be a compound is an alloy

Alloy

An alloy is a partial or complete solid solution of one or more chemical element in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal history....
. It is made mechanically, most commonly by heating up all of the constituent(s) and then cooling it quickly so that the constituents are then "caught" in the base metal.

Nevertheless, there are multiple border cases between compound and mixture. Examples are solutions of alkali metals in liquid ammonia, intermetallic compounds, etc..

Formula

Chemists describe compounds using formula in various formats. For molecules, the formula for the molecular unit is shown. For polymeric materials

Polymer

A polymer is a large molecule composed of repeating structural units typically connected by covalent chemical bonds. While polymer in popular usage suggests plastic, the term actually refers to a large class of natural and synthetic materials with a variety of properties....
, such as mineral

Mineral

A mineral is a naturally occurring solid formed through Geology processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties....
s and many metal

Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
oxide

Oxide

An oxide is a chemical compound contaning at least one oxygen atom as well as at least one other element. Most of the Earth's crust consists of oxides....
s, the empirical formula is given, e.g. NaCl for table salt. The order of the elements in molecular and empirical formulas is C, then H and then alphabetical. Trifluoroacetic acid

Trifluoroacetic acid

Trifluoroacetic acid is the most simple perfluorinated carboxylic acid chemical compound with the formula CF3CO2H. It is a strong carboxylic acid due to the influence of the three very electronegativity fluorine atoms....
is thus described as C₂HF₃O₂. More descriptive formulas convey structure information, illustrated again with trifluoroacetic acid. CF₃CO₂H. On the other hand, formulas for inorganic compound

Inorganic compound

Traditionally, inorganic compounds are considered to be of a mineral, not biological, origin. Complementarily, most organic compounds are traditionally viewed as being of biological origin....
s often do not convey structural information, as illustrated by H₂SO₄ for a molecule that has no H-S bonds. A more descriptive presentation would be O₂S(OH)₂.

Elements form compounds to become more stable. They become stable when they have the maximum number of possible electrons in their outermost energy level, which is normally two or eight valence electrons. This is the reason that noble gases do not frequently react: they already possess eight valence electrons (the exception being helium

Helium

Helium is a colorless, odorless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas group in the periodic table and whose atomic number is 2....
, which requires only two valence electrons to achieve stability).

Phases and thermal properties

Compounds may have several possible phases. All compounds can exist as solid

Solid

A solid object is in the states of matter characterized by resistance to deformation and changes of volume. In other words, it has high values both of Young's modulus and of shear modulus; this contrasts e.g....
s, at least at low enough temperatures. Molecular compounds may also exist as liquid

Liquid

Liquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material....
s, gas

Gas

In physics, a gas is a state of matter, consisting of a collection of particles without a definite shape or volume that are in more or less random motion....
es, and, in some cases, even plasmas

Plasma (physics)

In physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule....
. All compounds decompose upon applying heat

Heat

In physics and thermodynamics, heat is any transfer of energy from one body or thermodynamic system to another due to a difference in temperature....
. The temperature

Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
at which such fragmentation occurs is often called the decomposition temperature. Decomposition temperatures are not sharp and depend on the rate of heating. At sufficiently high temperatures, all compounds, either after they have decomposed somehow or in the act of decomposing, fragment into smaller compounds or to individual atom

Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
s.

CAS number

Every chemical compound that has been described in the literature carries a unique numerical identifier

Identifier

In computer science, Identifiers are Lexical Token s that name entity. The concept is analogy to that of a "name". Identifiers are used extensively in virtually all information processing systems....
, its CAS number.