Question

How do you calculate lone pairs?

Answer 1

Lone pairs are the non-bonding electrons that are not involved in the participation of a chemical bond and will exist freely on the atoms in the molecule. Two non-bonding electrons can be called a one lone pair.

Complete answer:
Molecules consist of the combination of atoms combined to each other through a chemical bonding. The bonding involved may be an ionic or covalent bond character depending upon the electrons sharing or transfer between two atoms.
When the molecule consists of two atoms, one electron from each atom is involved in bonding depending upon the valency of the atom.
Let us take an example of water, the central metal atom is oxygen, and the atoms bound to oxygen are two hydrogen atoms.
Oxygen has $8$ electrons and Hydrogen has $1$ electrons.
The valence electrons of Oxygen are $6$ , and the valence electrons on the two hydrogen atoms are $2$ . Thus, the total valence electrons on the water molecule will be $8$.
Out of these $8$ electrons, two electrons from an oxygen atom and two electrons from two hydrogen atoms involved in chemical bonding. Thus, the water molecule has a total of $4$bonding electrons. The remaining $4$ electrons will exist as a lone pair of electrons on a central metal atom i.e.., Oxygen atom.
Thus, the lone pairs are $2$ .

Note:
The lone pairs exist on the atoms, mostly central metal atoms and freely exist; the presence of lone pairs of electrons makes the molecule more basic, as the molecule is ready to donate the pair of electrons which can be considered as a Lewis base.