How is shielding effect explained?

Hint: The shielding effect is observed by the valence electrons exerted by the nucleus which is the decrease in the force of attraction due to the presence of electrons in the inner shell.

Complete answer:
In an atom containing multielectron, the electrons present in the valence shell are attracted towards the nucleus and these electrons are repelled by the electrons which are repelled by the electrons allocated in the inner shell. Due to this the actual force of attraction between the nucleus and the valence electrons is reduced by the repulsive force which is acting in the opposite direction.
This decrease in the force of attraction exerted by the nucleus on the valence electrons resulting from the presence of electrons from the availability of electrons in the inner shells is known as shielding effect.
The shielding effect occurs between the sublevels between the same principal energy level. An electron in the s sublevel shields electrons present in the p sublevel of the same principal energy level. This is due to the spherical shape of the s orbital.
Shielding effect is explained by taking an example of a lithium atom. A Lithium atom contains three protons and three electrons. The electronic configuration of lithium is $1{s^2}2{s^1}$, two electrons are present in the first principal energy level and one electron (valence electron) in the second principal energy level. The valence electron is partially shielded by the attractive force of the nucleus by the two electrons present in the inner shell. The removal of the valence electrons becomes easy due to shielding effect.

Note: The shielding effect is also known as screening effect. As electrons in s orbital can shield the electrons of p-orbital but the electrons present in the p-orbital do not shield the electrons present in s-orbital.