Please explain shielding effect .

In multi-electron atoms, the electrons present in the outermost shell do not experience the complete attraction by the nuclear charge because of the inner electrons. In other words, the force of attraction experienced by the valence electrons is less than that experienced by the inner electrons. Thus, the outermost electrons are shielded or screened from the nucleus by the inner electrons. This is known as shielding effect or screening effect.

Poor shielding therefore means poor screening of nuclear charge. In other words, the nuclear charge is not effectively screened by electrons in question.

1. - s orbitals have the largest screening effect for a given n value since s electrons are closer to the nucleus.
2. - p orbital's have the next highest screening effect and then comes d and then f orbital's. Thus in simple terms the screening effect decreases as

s orbitals > p orbitals> d orbitals> f orbitals