Question
Ionisation enthalpy $$\left( {{\Delta _i}{H_1}\,kJ\,mo{l^{ - 1}}} \right)$$ for the elements of Group 13 follows the order
A.
$$B > Al > Ga > In > Tl$$
B.
$$B < Al < Ga < In < Tl$$
C.
$$B < Al > Ga < In > Tl$$
D.
$$B > Al < Ga > In < Tl$$
Answer :
$$B > Al < Ga > In < Tl$$
Solution :
The decrease in ionisation enthalpy from $$B$$ to $$Al$$ is associated with increase in size. The observed discontinuity in the ionisation enthalpy values between $$Al$$ and $$Ga,$$ and between $$In$$ and $$Tl$$ are due to inability of $$d$$ - and $$f$$ - electrons, which have low screening effect, to compensate the increase in nuclear charge.