(a) How do atom exist?
(b) What is atomicity?
(c) What are polyatomic ions?

(a) Atom exists in two forms:

1. In the form of molecules - Molecules can be formed either by the combination of atoms of the same element or of different elements.

For ex: O₂ is formed by the combination of two Oxygen atoms.

2. In the form of ions - Ions are formed by the loss or gain of electrons by an atom.

For ex: common salt or sodium chloride compound is formed by the collection of millions of sodium ions (Na⁺) and chloride ions (Cl^-).

(b) Atomicity of an element is a measure of the total number of atoms present in a molecule.

For example, an oxygen molecule contains two oxygen atoms.

(c) Polyatomic ions are clusters or group of atoms that always have a fixed charge attach to them. This charge may be positive or negative.

 For example, NO₃^-