In multi-electron atoms, the electrons present in the outermost shell do not experience the complete nuclear charge because of repulsive interaction of the intervening 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.
Penetration Power means the ability of an orbital to attract an electron. This process is accompanied with release of energy. The Penetration effect of s orbital is the maximum because of the closeness to the nucleus than are the p, d and f orbitals. Due to greater penetration effect of s orbital, it releases greater amount of energy than p orbital when it accepts an electron. Similarly the energy released by a d orbital is more than that of a f orbital but less than that of a p orbital.
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