Atomic mass- The atomic mass (A) of an element is defined as the average mass of an atom of an element in the atomic mass unit. The atomic mass of an element is the weighted average of the atomic masses of all the isotopes of an element.

A molecule is the smallest particle of an element or a compound that can exist in the free state independently under ordinary conditions and exhibits all the properties of the substance (element or compound). Example: H 2 , O 2 , etc.

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.