The number of atoms of all the elements present in a molecule of a substance is known as the atomicity of that molecule. For Example, the atomicity of hydrogen H 2 molecule is 2 (diatomic), atomicity of NH 3 is 4 (tetra atomic).

The charged species formed when an atom gains or loses electrons is called an ion. A positively charged ion is called a cation and a negatively charged ion is called an anion. A group of atoms carrying charge is called a polyatomic ion.

Valency of an atom or radical is the number of electrons an element loses or gains or shares during the course of a chemical reaction. Therefore, valency is the combining capacity of an element. For example, the valency of sodium, magnesium, and aluminium is 1 , 2 , and 3 respectively.