(i) Relative atomic mass: The relative atomic mass of an element is defined as the average mass of the atom, as compared to 1/12^ththe mass of one-carbon atom.

The relative atomic masses of all the elements have been found with respect to an atom of carbon-12.

(ii) Atomic mass unit: One atomic mass unit is a mass unit equal to the exactly 1/12^th the mass of one atom of carbon-12.

Carbon-112 was chosen as the standard reference for measuring atomic masses.

(iii) Ions: Compounds composed of metals and non-metals has charged species. The charge species are known as ions.

The negatively charged ions are called anions and positively charged ions are called cations.

(iv) Ionic compounds: Compounds formed by the exchange of ions (cations or anions) are called ionic compounds.

In ionic compounds, the charge on each ion is used to determine the chemical formula of the compound.

(v) Atomicity:

Atomicity: The number of atoms constituting a molecule is known as its atomicity. For example:

In P₄, there are four atoms of phosphorus constituting a molecule. Hence, the atomicity is 4 (tetra-atomic).