(a) Define ionization energy.
(b) Prove that ionization energy is a periodic property.

Explanation

(a) The energy required to remove the most loosely held electron from an isolated gaseous atom is called as ionization energy.
(b) 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.