When the number of inner electrons is greater, they shield the outermost electron from the nucleus so that the outermost electron becomes free from any nuclear attraction. This is called the shielding or screening effect.
Screening effect or shielding effect
In a multielectron atom, the valence shell’s electrons are attracted to the nucleus, and these electrons are repelled by the electrons present in the inner shells. On account of this, the actual force of attraction between the nucleus and the valence electrons is somewhat decreased by the repulsive forces acting in opposite directions. This decrease in the force of attraction exerted by the nucleus on the valence electrons due to the presence of electrons in the inner shells is called screening effect or shielding effect.
The magnitude of the screening effect depends upon the number of inner electrons higher the number of inner electrons greater shall be the value of the screening effect. The symbol σ represents the screening effect constant.