The mass number of an atom is 31. The M shell of this atom contains 5 electrons.

Write the electronic configuration.

The mass number of the atom is 31.

We know that mass number = number of protons + number of neutrons.

The M shell of this atom contains 5 electrons.

This condition could only be satisfied when the M shell has the following electronic

Configuration :- 3s² 3p³ (5 electrons)

Upon filling the subsequent inner shells we have the following electronic configuration –

K shell - 2 electrons

L shell – 8 electrons

M shell – 5 electrons

Hence, the total number of electrons is 15.