Meaning of Screening or Shielding Effect.
Screening or shielding effect is due to shielding of nuclear charge by inner shell electrons.
More are the number of inner shell electrons; more is the nuclear charge screening and lesser the positive charge felt by valence electrons.
For example, Na has 10 electrons screening its outermost shell electrons and potassium has 18 electrons screening its outermost shell. So, screening effect is more on valence electron of potassium then sodium.
K -[Ar] 4s1
Na -[Ne] 3s1
K has more screening effect than sodium. Down the group Screening or shielding effect increases.
It means valence electron of potassium is less tightly bound to the nucleus due to more screening of nuclear charge and less attraction with the nucleus.
The presence of shielding electrons reduces the electrostatic attraction between the positively charged protons in the nucleus and the outer electrons. Moreover, the repulsive forces between electrons in a many-electron atom further offset the attractive force exerted by the nucleus.
Screening or Shielding Effect Across the Period.
Across a period, the number of electron shells remains constant, so the number of inner-shell electrons does not change. Consequently, the shielding effect remains relatively constant. For example, Na has 10 electrons screening its outermost shell electrons and magnesium has 1o electrons screening its outermost shell. So, screening effect is same on valence electron
Shielding Effect Down the Group.
Down the group, the number of electron shells are increasing, so the number of inner-shell electrons are also increasing. Consequently, the shielding effect remains increasing. For example, Mg has 10 electrons screening its outermost shell electrons and Calcium has 18 electrons screening its outermost shell. So, screening effect is more on valence electron of potassium then sodium.
Penetration Effects of the Orbitals-
Penetration effects of different orbitals can be understood from the probability distribution curves of the electrons in these orbitals. It has been observed that penetration effect follows this order-
s> p> d> f
More is the penetration effect of the orbitals, more closer is the electron to nucleus which means more attraction between the electron and the nucleus and less energy.
