Formation of Ion

1. An ion is an atom or group of atoms carrying positive or negative charge. Example Ca²⁺, O^2-, SO₄^2- etc.
2. If a particle has equal numbers of protons (+) and electrons (-), the particle charge is zero, and the particle is said to be neutral.
3. In a chemical reaction, electron(s) can be transferred from atom to another atom.
4. If electron(s) is removed from an atom, the number of protons will be more than number of electrons. In this case, the atom will has excess positive charge and hence form a positive ion (cation).

   

   (An atom losses electron to form positive ion)

5. If an atom gains electrons, there is an excess negative charge in the atom, so a negative ion is formed.
6. In other words,
1. The atom losing electrons forms a positive ion (cation) and is usually a metal.
2. The atom gaining electrons forms a negative ion (anion) and is usually a non-metallic element.

(An atom gain electron to form negative ion)