# What is a compound?

Jul 5, 2017

A compound is a pure substance that is composed of two or more elements.

#### Explanation:

A compound is a pure substance that is composed of two or more elements. Some compounds are formed by covalent bonding, in which valence electrons are shared between atoms; and some compounds are formed by ionic bonding, in which the complete transfer of one or more electrons from one atom to another occurs.

The smallest particle of a covalent compound is called a molecule. Because ionic compounds form crystal lattices, they do not exist as molecules. Instead, the smallest whole number ratio that exists between the cations and anions is called a formula unit.

All compounds have a definite composition. For example, a compound of hydrogen and oxygen that forms water has the formula $\text{H"_2"O}$. However, hydrogen peroxide is also composed of hydrogen and oxygen with the chemical formula ${\text{H"_2"O}}_{2}$. Both are molecular compounds, but their composition is different, and water and hydrogen peroxide have very different properties.

The diagram below represents a molecule of methane gas $\left(\text{CH"_4}\right)$. Each hydrogen atom shares its valence electron with one of the carbon atoms valence electrons. There are a total of four covalent bonds in this molecule.

The diagram below shows part of a crystal lattice of sodium chloride $\left(\text{NaCl}\right)$, an ionic compound composed of sodium cations $\left({\text{Na}}^{+}\right)$ and chloride anions $\left({\text{Cl}}^{-}\right)$.