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A chemical compound is a
chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which ...
composed of many identical
molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms held together by chemical bo ...

molecule
s (or molecular entities) composed of
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

atom
s from more than one
element Element may refer to: Science * Chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all ...
held together by
chemical bond A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
s. A molecule consisting of atoms of only one element is therefore not a compound. There are four types of compounds, depending on how the constituent atoms are held together: *
molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms held together by chemical bo ...

molecule
s held together by
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they sh ...
s *
ionic compound structure of sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salt (chemistry), salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chlori ...
s held together by
ionic bond Ionic bonding is a type of chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of attraction be ...
s *
intermetallic compounds An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy An alloy is an admixture of metal A metal (from Ancient Greek, Gree ...
held together by
metallic bond Metallic bonding is a type of chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of att ...
s * certain complexes held together by
coordinate covalent bondA coordinate covalent bond, also known as a dative bond, dipolar bond, or coordinate bond is a kind of two-center, two-electron covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These ele ...
s. A
chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes ...
specifies the number of atoms of each element in a compound molecule, using the standard abbreviations for the chemical elements and numerical
subscript Pro; the size of the subscript is about 62% of the original characters, dropped below the baseline by about 16%. The second typeface is Myriad A myriad (from Ancient Greek Ancient Greek includes the forms of the Greek language used in ...
s. For example, a
water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts ...

water
molecule has formula H2O indicating two
hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

hydrogen
atoms bonded to one
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

oxygen
atom. Many
chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wit ...

chemical
compounds have a unique
CAS number A CAS Registry Number, also referred to as CASRN or CAS Number, is a unique numerical identifier assigned by the Chemical Abstracts Service (CAS) to every chemical substance described in the open scientific literature (currently including all su ...
identifier assigned by the
Chemical Abstracts Service Chemical Abstracts Service (CAS) is a division of the American Chemical Society The American Chemical Society (ACS) is a scientific society A learned society (; also known as a learned academy, scholarly society, or academic association) is ...
. Globally, more than 350,000 chemical compounds (including mixtures of chemicals) have been registered for production and use. A compound can be converted to a different chemical substance by interaction with a second substance via a
chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the posit ...

chemical reaction
. In this process, bonds between atoms may be broken in either or both of the interacting substances, and new bonds formed.


Definitions

Any substance consisting of two or more different types of
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

atom
s (
chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nu ...
s) in a fixed
stoichiometric Stoichiometry is the calculation of reactants and products in chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical s ...
proportion can be termed a ''chemical compound''; the concept is most readily understood when considering pure
chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which ...
s. It follows from their being composed of fixed proportions of two or more types of atoms that chemical compounds can be converted, via
chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the posit ...

chemical reaction
, into compounds or substances each having fewer atoms. The ratio of each element in the compound is expressed in a ratio in its chemical formula. A
chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes ...
is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using the standard abbreviations for the chemical elements, and
subscript Pro; the size of the subscript is about 62% of the original characters, dropped below the baseline by about 16%. The second typeface is Myriad A myriad (from Ancient Greek Ancient Greek includes the forms of the Greek language used in ...
s to indicate the number of atoms involved. For example,
water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts ...

water
is composed of two
hydrogen atom A hydrogen atom is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touch ...

hydrogen atom
s bonded to one
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

oxygen
atom: the chemical formula is H2O. In the case of non-stoichiometric compounds, the proportions may be reproducible with regard to their preparation, and give fixed proportions of their component elements, but proportions that are not integral .g.,_for_palladium_hydride,_PdHx_(0.02_<_x_<_0.58).html" ;"title="palladium_hydride.html" ;"title=".g., for palladium hydride">.g., for palladium hydride, PdHx (0.02 < x < 0.58)">palladium_hydride.html" ;"title=".g., for palladium hydride">.g., for palladium hydride, PdHx (0.02 < x < 0.58)Phase diagram for Palladium-Hydrogen System
/ref> Chemical compounds have a unique and defined chemical structure held together in a defined spatial arrangement by
chemical bond A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday ...
s. Chemical compounds can be
molecular A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atom An atom is the smallest unit of ordinary matter In ...

molecular
compounds held together by
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they sh ...
s,
salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of cations and anions. Salts are composed of related numbers of cations (positively electric charge, charged ions) and anions (negatively charged ions) so that the prod ...
held together by
ionic bond Ionic bonding is a type of chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of attraction be ...
s,
intermetallic compounds An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy An alloy is an admixture of metal A metal (from Ancient Greek, Gree ...
held together by
metallic bond Metallic bonding is a type of chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of att ...
s, or the subset of chemical complexes that are held together by
coordinate covalent bondA coordinate covalent bond, also known as a dative bond, dipolar bond, or coordinate bond is a kind of two-center, two-electron covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These ele ...
s. Pure
chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nu ...
s are generally not considered chemical compounds, failing the two or more atom requirement, though they often consist of molecules composed of multiple atoms (such as in the
diatomic molecule Diatomic molecules are molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more ...
H2, or the polyatomic molecule S8, etc.). Many
chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wit ...

chemical
compounds have a unique numerical identifier assigned by the
Chemical Abstracts Service Chemical Abstracts Service (CAS) is a division of the American Chemical Society The American Chemical Society (ACS) is a scientific society A learned society (; also known as a learned academy, scholarly society, or academic association) is ...
(CAS): its
CAS number A CAS Registry Number, also referred to as CASRN or CAS Number, is a unique numerical identifier assigned by the Chemical Abstracts Service (CAS) to every chemical substance described in the open scientific literature (currently including all su ...
. There is varying and sometimes inconsistent nomenclature differentiating substances, which include truly non-stoichiometric examples, from chemical compounds, which require the fixed ratios. Many solid chemical substances—for example many silicate minerals—are chemical substances, but do not have simple formulae reflecting chemically bonding of elements to one another in fixed ratios; even so, these Crystal structure, crystalline substances are often called " non-stoichiometric compounds". It may be argued that they are related to, rather than being chemical compounds, insofar as the variability in their compositions is often due to either the presence of foreign elements trapped within the crystal structure of an otherwise known true ''chemical compound'', or due to perturbations in structure relative to the known compound that arise because of an excess of deficit of the constituent elements at places in its structure; such non-stoichiometric substances form most of the crust (geology), crust and mantle (geology), mantle of the Earth. Other compounds regarded as chemically identical may have varying amounts of heavy or light isotopes of the constituent elements, which changes the ratio of elements by mass slightly.


Types


Molecules

A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. A molecule may be homonuclear, that is, it consists of atoms of one chemical element, as with two atoms in the
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

oxygen
molecule (O2); or it may be heteronuclear, a chemical compound composed of more than one element, as with water (molecule), water (two hydrogen atoms and one oxygen atom; H2O).


Ionic compounds

An ionic compound is a chemical compound composed of ions held together by Coulomb's law, electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions such as the sodium (Na+) and chloride (Cl) in sodium chloride, or Polyatomic ion, polyatomic species such as the ammonium () and carbonate () ions in ammonium carbonate. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network, usually in a Crystal structure, crystalline structure. Ionic compounds containing basic ions hydroxide (OH) or oxide (O2−) are classified as bases. Ionic compounds without these ions are also known as
salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of cations and anions. Salts are composed of related numbers of cations (positively electric charge, charged ions) and anions (negatively charged ions) so that the prod ...
and can be formed by Acid–base reaction#Arrhenius theory, acid–base reactions. Ionic compounds can also be produced from their constituent ions by evaporation of their solvent, precipitation (chemistry), precipitation, freezing, a solid-state reaction route, solid-state reaction, or the electron transfer reaction of Reactivity series, reactive metals with reactive non-metals, such as halogen gases. Ionic compounds typically have high Melting point, melting and boiling points, and are Hardness, hard and Brittleness, brittle. As solids they are almost always insulator (electricity), electrically insulating, but when melting, melted or Dissolution (chemistry), dissolved they become highly electrical resistivity and conductivity, conductive, because the ions are mobilized.


Intermetallic compounds

An intermetallic compound is a type of metallic bonding, metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties. They can be classified as stoichiometric or nonstoichiometic intermetallic compounds.


Complexes

A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many metal-containing compounds, especially those of transition metals, are coordination complexes. A coordination complex whose centre is a metal atom is called a metal complex of d block element.


Bonding and forces

Compounds are held together through a variety of different types of bonding and forces. The differences in the types of bonds in compounds differ based on the types of elements present in the compound. London dispersion forces are the weakest force of all intermolecular forces. They are temporary attractive forces that form when the electrons in two adjacent atoms are positioned so that they create a temporary dipole. Additionally, London dispersion forces are responsible for condensing chemical polarity, non polar substances to liquids, and to further freeze to a solid state dependent on how low the temperature of the environment is. A
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they sh ...
, also known as a molecular bond, involves the sharing of electrons between two atoms. Primarily, this type of bond occurs between elements that fall close to each other on the periodic table of elements, yet it is observed between some metals and nonmetals. This is due to the mechanism of this type of bond. Elements that fall close to each other on the periodic table tend to have similar electronegativity, electronegativities, which means they have a similar affinity for electrons. Since neither element has a stronger affinity to donate or gain electrons, it causes the elements to share electrons so both elements have a more stable octet rule, octet. Ionic bonding occurs when valence electrons are completely transferred between elements. Opposite to covalent bonding, this chemical bond creates two oppositely charged ions. The metals in ionic bonding usually lose their valence electrons, becoming a positively charged cation. The nonmetal will gain the electrons from the metal, making the nonmetal a negatively charged anion. As outlined, ionic bonds occur between an electron donor, usually a metal, and an electron acceptor, which tends to be a nonmetal. Hydrogen bonding occurs when a
hydrogen atom A hydrogen atom is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touch ...

hydrogen atom
bonded to an electronegative atom forms an electrostatic connection with another electronegative atom through interacting dipoles or charges.


Reactions

A compound can be converted to a different chemical composition by interaction with a second chemical compound via a
chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the posit ...

chemical reaction
. In this process, bonds between atoms are broken in both of the interacting compounds, and then bonds are reformed so that new associations are made between atoms. Schematically, this reaction could be described as , where A, B, C, and D are each unique atoms; and AB, AD, CD, and CB are each unique compounds.


See also

* Chemical structure * IUPAC nomenclature * Dictionary of chemical formulas * List of compounds


References


Further reading

* {{Authority control Chemistry Chemical compounds,