The more shielding means less Zeff, less Zeff means less attraction between nucl

Question

The more shielding means less Zeff, less Zeff means less attraction between nucleus and electron, less attraction means easy to remove an electron, there it is less ionization energy too?
But what about the exception between N and O, the half-filled and full-filled? N has larger Ionization energy than O, but does it have larger or lesser Zeff than O? The more shielding means less Zeff, less Zeff means less attraction between nucleus and electron, less attraction means easy to remove an electron, there it is less ionization energy too?
But what about the exception between N and O, the half-filled and full-filled? N has larger Ionization energy than O, but does it have larger or lesser Zeff than O?
But what about the exception between N and O, the half-filled and full-filled? N has larger Ionization energy than O, but does it have larger or lesser Zeff than O?

Explanation / Answer

Zeff is less with more shielding. So, as Zeff increases, the force between the nucleus and the valence electrons increases and hence the electrons are pulled closer to the nucleus. Ionisation energy is the minimum energy needed to remove a mole of electrons from a mole of gaseous atoms. Hence, as Zeff increases, the ionization energy also increases.

Between ‘O’ and ‘N’, ‘O’ has a lower ionization energy than ‘N’ due to an electron being added to an already half full orbital in oxygen, which results in electron electron repulsion. This makes it easier to remove an electron and hence will lower the ionisation energy. This can be rationalized based on their electronic configuration (shown below):

Electronic configuration of nitrogen : 1s22s22p3

Electronic configuration of oxygen : 1s22s22p4

Based on the fact, as Zeff increases the ionization energy also increases ; therefore, nitrogen will have larger Zeff than oxygen because of larger ionization energy of nitrogen.

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