(i) Variation in a period:
On moving across a period from left to right, the ionization enthalpy value increases.
This is due to the following reasons:
- Increase of nuclear charge in a period
- Decrease of atomic size in a period
Because of these reasons, the valence electrons are held more tightly by the nucleus, thus ionization enthalpy increases.
Hence, ionization energy is a periodic property.
(ii) Variation in a group:
As we move from top to bottom along a group, the ionization enthalpy decreases.
This is due to the following reasons:
- A gradual increase in atomic size
- Increase of screening effect on the outermost electrons due to the increase of number of inner electrons.
Hence, ionization enthalpy is a periodic property.
