A pair of electrons not shared with any other atom for bond formation is called a lone pair of electrons. In water, the central atom – oxygen has two lone pair of electrons as shown ahead:
Structure of water molecule
Formation of hydronium ion- (H3O+): When an acid is dissolved in water the proton (H+ ) released by the acid add onto the lone pair electrons of the oxygen atom of a water molecule. The proton (H+ ) accepts the lone pair of electrons forming a coordinate bond (shown by an arrow).
HCl → H+ + Cl–
Water Proton Hydronium ion
Formation of ammonium ion (NH4+): When ammonia gas is dissolved in water, the proton released by water adds onto the lone pair of electrons of the nitrogen atom of the ammonia molecule. The proton (H+) accepts the lone pair of electrons forming a coordinate bond (shown by an arrow).
