What is screening effect ? How …

The actual charge felt by the valence shell electron is called effective nuclear energy. It is denoted by 

z_eff  and the repulsive forces felt by valence shell electrons from the electron present in the inner shell is called shielding effect or screening effect.
z_eff = total nuclear charge − screening constant (σ)
Screening constant = total nuclear charge − z_eff
Greater the number of electrons in inner shells larger will be the screening effect. With increase in screening effect z_eff will be decreased.
As the force of attraction by the nucleus for the valence shell electrons decreases and hence the ionization energy decreases. In other words an increase in the number of electrons in the inner shell leads to decrease the ionization energy.

 

The electrons present in the inner shells act as screens between the nucleus and the valence shell electrons. In order words, these electrons partially neutralise the force fo attraction fo the nucleus, over the valence electrons. This is called, 'shielding or screening effect'.
When the screening effect increase (No. of inner electron-shells increase), the IP decreases.
THe magnitude of screening effect  ∝1/I.P.