(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.
