. Ionisation enthalpy

Ionisation enthalpy (Δ H1 kJ/mol) 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


Best Answer


First Ionization enthalpy is defined as the enthalpy required to remove the first electron from the isolated gaseous atom.

The ionization enthalpy depends on various factors including the penetrating effect, shielding effect and electronic configuration.

Generally, the ionization enthalpy increases along the period and decreases down the group.

But, on moving down Group 13, a regular decrease in ionization enthalpy is not observed.

On moving from B to Al, the Ionization enthalpy decreases because the atomic size increases ( as expected)

B > Al

But as we move from Al to Ga, the ionization enthalpy slightly increases, because of the increase in effective nuclear charge and shielding effect by d-electrons present in Ga.

Al < Ga

The d-electrons provide poor shielding towards the outermost electron and hence, because of the poor shielding and increased effective nuclear charge, its ionization enthalpy increases.

On moving further, the ionization enthalpy decreases for In because by now the shielding by d-electrons have overcome the effective nuclear charge.

Ga > In

On moving from In to Tl, we see an increase in the ionisation enthalpy because of the increased effective nuclear charge and the presence of f-electrons.

The f-electrons shield the outermost electron poorly and hence, the effective nuclear charge overpowers the shielding effect, making it difficult to extract electron and hence, the ionization enthalpy increases.

In < Tl

The ionization enthalpies observed are:

Element B Al Ga In Tl
Δ i H1 kJ/mol 801 577 579 558 589

Final Answer:

Hence, the correct answer is option D: B > Al < Ga > In < Tl


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