What is Atomicity?

The atomicity of a substance is the number of atoms that
each molecule of the substance has.

To illustrate this with
an example, consider the reaction between hydrogen and oxygen to form
water.

2H2 + O2 -->
2H2O

Two molecule of hydrogen react with one molecule of
oxygen to form two molecules of water. Each molecule of hydrogen has two atoms of
hydrogen, the atomicity of hydrogen is two. Similarly each molecule of oxygen contains
two atoms of oxygen, the atomicity of oxygen is two. One molecule of water contains two
atoms of hydrogen and one atom of oxygen, the atomicity of water is
three.

In gases, mono-atomic gases have molecules that are
made of only one atom, this is shown by the inert gases. Diatomic gases have two atoms
in each molecule, examples are hydrogen, carbon monoxide, chlorine, etc. Triatomic gases
have three atoms in a molecule, examples are ozone, nitrogen dioxide, carbon dioxide,
etc.