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A [[chemical compound]] is any substance that consists of two or more [[chemical element]]s always associated with one another in the same proportion, and that has certain properties deriving from that association that define and distinguish it from other substances.<ref>Wile, Dr. Jay L. ''Exploring Creation With Chemistry''. Apologia Educational Ministries, Inc. 1998</ref>

A compound is one example of a [[pure substance]]. (The other is a [[chemical element]].)

== Types of Compounds ==
Two types of chemical compounds are known to [[chemistry]]. They are ''molecular compounds'' and ''ionic compounds.''

=== Ionic Compound ===
An ''ionic compound'' is any substance consisting of two or more [[ionic bond|ionically-bonded]] atoms.

Table salt ([[sodium chloride]]) is a good example of an ionic compound. As a pure solid, salt consists of [[sodium]] and [[chlorine|chloride]] atoms tightly associated in a crystal lattice. When salt dissolves, these atoms [[dissociate]] and become sodium ions and chloride ions, distributed throughout the [[solvent]]. No such thing as "a molecule of salt" can ever be said to exist (since salt is a crystal), however one NaCl unit is referred to as a ''formula unit.''

Ionic compounds typically dissolve only in ''polar'' solvents (see below).

=== Molecular Compound ===
A ''molecular compound'' is a compound which is composed of atoms joined by [[covalent bond]]s. In contrast to the formula unit of ionic crystals, the basic unit of a molecular compound is the ''molecule''. A molecule is the smallest particle of a compound that retains the unique defining properties of that compound.

Molecules may be either ''polar'' or ''nonpolar''. Polar molecules have an unequal distribution of electric charge among their atoms, giving them distinct positive and negative regions. The charge in any region of a polar molecule is almost never equal to the full charge on, say, a single [[proton]] or [[electron]]. If it were, then the particle would be an ion. Nonpolar molecules distribute their electrons evenly and thus have no partial positive or partial negative charge on any aspect of them.

In general, polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents. Ionic compounds (see above) typically dissolve only in polar solvents.

Molecular compounds are also either ''organic'' or ''inorganic''. [[Organic compound|Organic compounds]] contain carbon; inorganic compounds do not. Polyatomic ions may also be organic or inorganic -- though an organic polyatomic ion is usually a dissociated weak organic acid of the form R-COOH, where R is the rest, or ''residue'', of the molecule.

== Descriptors of Compounds ==
=== Chemical Formula ===
: ''Main Article: [[Chemical formula]]''

The ''chemical formula'' of any compound (ionic or molecular) is either a list of the [[Chemical symbol|chemical symbols]] of its constituent elements, with subscripts to describe their ratio, or else a slightly more complex listing or even a drawing intended to describe not merely the ratio of the constituent elements but also their arrangement. A chemical compound might have as many as three types of formulas.

==== Simple formula ====
The ''simple formula'' simply lists the elements in symbolic form, with subscripts to describe their ratio. For example, [[water]] consists of two parts [[hydrogen]] and one part [[oxygen]]. Its simple formula is, therefore, H<sub>2</sub>O, that of the basic solute of common household vinegar ([[acetic acid]]) is C<sub>2</sub>H<sub>4</sub>O<sub>2</sub> (This is often written as CH<sub>3</sub>COOH to better reflect its structure), and that of common salt is NaCl. One usually describes an ionic compound only by a simple formula, though exceptions do exist.

==== Graphic formula ====
The ''graphic formula'' is a one-line list of the elements intended to show the physical orientation of the constituent atoms to one another. For example, the graphic formula of water is HOH. From this rendering, one can readily see that the water molecule has an atom of hydrogen to either side of an atom of oxygen. (This is a slight oversimplification.) The graphic formula of acetic acid is CH<sub>3</sub>COOH, and from this formula one can picture the acetic acid molecule as consisting of one atom of carbon, with three atoms of hydrogen bound to it, and also bound to another atom of carbon that in turn has two atoms of oxygen bound to it, one of which has an atom of hydrogen bound on its other side.

==== Structural formula ====
The ''structural formula'' is an often elaborate two-dimensional drawing showing the relationship of the various atoms to one another. The structural formula for water would be H-O-H if one habitually used a structural formula to describe water.

The most elaborate type of structural formula would mention each and every atom in the molecule, including hydrogen. But commonly used structural formulas for organic compounds typically leave off any hydrogen atoms bound to carbon (and assume that hydrogen atoms are bound to any vacant positions on any carbon atoms), and represent the carbon atoms as corners in an angular drawing of the backbone of the molecule. Thus a simplified structural formula for acetic acid might look like the one at left.

Here one assumes that a carbon atom resides at the end of the backbone and at the center, with three hydrogen atoms occupying the vacant spaces on the carbon atom at the end.

Organic compounds have graphic and structural formulas; such formulas are usually unnecessary to the description of inorganic compounds.

==== SMILES ====
: Main Article: [[Simplified molecular input line entry specification|SMILES]]

The  '''simplified molecular input line entry specification''' is a one-line specification of a structural formula that does not require any special data types or drawings. For details, see the linked main article. The SMILES for [[carbon dioxide]] is O=C=O, and that for acetic acid is CC(=O)O. The SMILES for sodium chloride is <nowiki>[Na+].[Cl-]</nowiki>, a form that emphasizes the character of this compound as a crystal or other pairing of disconnected ions.

=== Formula Mass ===
The ''formula mass'' of any compound (ionic or molecular) is equal to the sum of the [[atomic mass|atomic masses]] of its constituent elements. If the compound is molecular, then the terms ''molecular mass'' or ''molar mass'' may properly describe this quantity.

==References==
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[[Category:Chemistry]]
[[Category:Chemical Compounds]]