Compound Definition in Chemistry

The word "compound" has several definitions. In the field of chemistry, "compound" refers to a "chemical compound."

Compound Definition

A compound is a chemical species that is formed when two or more atoms join together chemically, with covalent or ionic bonds.

Compounds may be categorized according to the type of chemical bonds holding the atoms together:

• Molecules are held together by covalent bonds.
• Ionic compounds are held together by ionic bonds.
• Intermetallic compounds are held together by metallic bonds.
• Complexes are often held together by coordinate covalent bonds.

Note that some compounds contain a mixture of ionic and covalent bonds. Also note, a few scientists do not consider pure elemental metals to be compounds (metallic bonds).

Examples of Compounds

Examples of compounds include table salt or sodium chloride (NaCl, an ionic compound), sucrose (a molecule), nitrogen gas (N₂, a covalent molecule), a sample of copper (intermetallic), and water (H₂O, a covalent molecule). Examples of chemical species not considered compounds include the hydrogen ion H⁺ and the noble gas elements (e.g., argon, neon, helium), which do not readily form chemical bonds.

Writing Compound Formulas

By convention, when atoms form a compound, its formula lists the atom(s) acting as a cation first, followed by the atom(s) acting as the anion. This means sometimes an atom may be first or last in a formula. For example, in carbon dioxide (CO₂), carbon (C) acts as a cation. In silicon carbide (SiC), carbon acts as the anion.

Compound Versus Molecule

Sometimes a compound is called a molecule. Usually, the two terms are synonymous. Some scientists make a distinction between the types of bonds in molecules (covalent) and compounds (ionic).