The amount of energy required to remove an electron from an isolated gaseous atom in the ground state is called ionization enthalpy.
- Ionization enthalpy increases along the period.
Reason: This is because along the period atomic size decreases and effective nuclear charge increases.
- Ionization enthalpy decreases down the group.
Reason: This is because atomic size increases down the group due to the increase in number of shells. The shielding effect increases and hence effective nuclear charge decreases.