Question

Why a higher energy is needed to remove the fourth electron?

Answer 1

Because carbon and nitrogen have four and five valence electrons, respectively, their fourth ionization energies correspond to removing an electron from a partially filled valence shell. ... This should require much more energy. The actual values are as follows: B, 25,026 kJ/mol; C, 6223 kJ/mol; and N, 7475 kJ/mol.1

Answer 2

A higher energy is needed to remove the fourth electron because the effective nuclear charge acting on the electrons increases.

Explanation:

• The removal of an electron from a neutral atom is easier.
• When one electron is removed, the atom changes into a cation.
• It is difficult to remove a negatively charged electron from a cation because after the removal of an electron, the effective nuclear charge acting on the other electrons increases.
• With the successive removal of electrons, this effective nuclear charge increases further.
• Therefore, it requires a huge amount of energy to remove the fourth electron as the electron is much more tightly bound to the nucleus in $\[{A^{3 + }}\]$ cation.