Question

What is meant by imperfect shielding of ${e^ - }$ by the nucleus?

Answer 1

Hint: Shielding effect is also known as screening which can be defined as the shielding of the outermost electrons from the nuclear force of attraction. This will lead the valence electrons to be loosely held by the nucleus and hence, the ionization energy will decrease as the electron can easily be removed from the atom.

Complete answer:
We know that there is a force of attraction between the nucleus and the electrons in an atom. But the inner electrons of an atom shield the outer or valence electrons and hence, less force of attraction between the nucleus and outermost electrons is observed. That is why the outer electrons are loosely bound and can easily be removed from the atom. This effect is known as the shielding effect. Shielding effect is inversely proportional to the ionization energy of the atom.
But the electrons present in d and f orbitals show poor shielding or we can say imperfect shielding due to which the net force of attraction (nuclear force) increases on the valence and outer electrons and the ionization energy of the atom increases.
Hence the decreasing order of the orbitals to shield the electrons is:
$s > p > d > f$

Note:
We should note that the ability of electrons in orbitals to shield the outermost electrons depends upon their size. S orbitals have highest shielding effect as it is spherical in shape and covers the nucleus while d and f orbitals shows imperfect shielding because they have double dumbbell shape.