Question

if the atomic number of element M is 20 then what will be the formula of its sulphite

Answer 1

Atomic number is 20 so electronic configuration would be 1s2 2s2 2p6 3s2 3p6 4s2.
The element (calcium) would lose 2 electrons to gain a stable octet configuration.

Hence M would become M+2 as Sulphite ion is SO3 -2
By criss cross method the formula of its Sulphite is

MSO3

Hope this helps you.

Answer 2

if the atomic no. of m is 20....the corresponding sulphite formed can be MSO3