The probability of an electron which allows an electron to get close to the nucleus is known as penetration effect. It is known as the proximity of the electron in the orbital to the nucleus.
(1) The relative penetration power of sub shells
within same shell (same value of n) follow the order as:
s>p>d>f
We consider it for each shell and sub shell as the relative density of the electrons near the nucleus of atoms. It is clear that the ‘S’ electrons have greater probability of coming closer to the nucleus than the P, d or f electrons of the same principal energy shell.
(2) For different values of shell
(n) and sub shell (l), decreasing penetrating power of an electron follows as:
1s>2s>2p>3s>3p>4s>3d>4p>5s>4d>5p>6s>4f....
(3) In other words, ‘S’ electrons penetrate (more nearer) more towards the nucleus than the ‘P’ electrons and the penetrating power of the electrons in the given principal energy shell varies as S> P> d>f. Thus the ‘S’ electrons experience more attraction from the nucleus than the p d or f electrons of the same principal energy shell.
Therefore, greater energy required to remove out electron from‘s’ orbital than ‘p’, d and ‘f’ orbital. Thus the ionization potential for pulling out an ‘s’ electron is maximum and it decreases in pulling out a p , d or f electron of the same principal energy shell.
(4) Ionization potential or Ionization enthalpy of an atom is directly proportional to penetration power of orbitals.
Application of Penetration effect:
Example: Ionization energy of Boron is smaller than Beryllium even though effective nuclear charge is higher?
Solution: The
electronic configurations of Boron and Beryllium are (5B=1S2,2S2,2p1)
and (4Be =1S2,2S2).
In Boron the
outermost electron is present in the 2p orbital (low penetration power) and is
less strongly bound than the electron present in a 2S orbital of Beryllium(Have
more penetration power), which will has a higher Zeff. It is easier to ionize
the Boron atom.
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