Explain the periodic trend of ionisation potential.

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.
Ionization energy is a periodic property. 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. Therefore, ionization enthalpy increases.
(c) 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