The following factors decrease the
ionisation enthalpy in the elements of the main group from top to bottom:
(i) Atomic size:
Atomic size increases due to addition of a new energy shell. This decreases
force of attraction towards nucleus and consequently, ionisation enthalpy
(ii) Screening effect: Due to increase of inner electrons,
the shielding effect increases which reduces force of attraction towards
nucleus and hence, ionisation enthalpy decreases.
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