Chemical compound

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A chemical compound is any substance that consists of two or more chemical elements 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.[1]

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 ionically-bonded atoms.

Table salt (sodium chloride) is a good example of an ionic compound. As a pure solid, salt consists of sodium and 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 composed of atoms joined by covalent bonds. 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 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 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, H2O, that of the basic solute of common household vinegar (acetic acid) is C2H4O2 (This is often written as CH3COOH 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 CH3COOH, 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.

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.


Main Article: 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 [Na+].[Cl-], 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 masses of its constituent elements. If the compound is molecular, then the terms molecular mass or molar mass may properly describe this quantity.

Molar mass

Molar mass is defined as mass of the substance of interest (the "solute") per unit volume of the "solvent" (whatever the substance is being measured with respect to) at a given temperature. Molality is defined as mass of solute per unit mass of the solvent, and is thus not affected by density changes.


  1. Wile, Dr. Jay L. Exploring Creation With Chemistry. Apologia Educational Ministries, Inc. 1998