Question

The first ionization enthalpy values (in kJ/mol) of group 13 elements are:
B= 801
Al= 577
Ga= 579
In=558
Tl=589
How would you explain this deviation from the general trend?

Answer 1

Hint:The ionization enthalpy is defined as the change in the enthalpy which is associated with the removal of the first electron from the isolated gaseous atom present in the ground state. So it requires energy to remove the electron from the isolated gaseous atom. It depends on the penetrating effect, shielding effect and the electronic configuration. The ionization enthalpy increases along the period while decreases down the group.

Complete answer:
So while we move down the group that is from B to Al the ionization enthalpy decreases because the atomic size increases as well as the screening effect also which has the ability to outweigh the increased effect of the nuclear charge. However the $${\mathrm{\Delta }}_{i}H$$for gas is slightly greater about 2kJ/mol then the $${\mathrm{\Delta }}_{i}H$$ for Al but it is much higher for the element Tl. So we can explain these deviation as the following: the element Al follows just after the s block elements while the element Ga and In follows after d block elements whereas Tl follows after the d and f block elements. These f shell electrons do not tend to shield the outermost shell electrons from the nucleus effectively. So due to this reason the valence electrons remains tightly held by the nucleus so large amount of energy is required for their removal. This shows and explains why Ga has higher ionization enthalpy than Al. now on moving further downward from Ga to In we see the increase in the shielding effect because of the presence of 4d electrons outweighs the increased nuclear charge effect(49-31=18units) and so the $${\mathrm{\Delta }}_{i}H$$ of IN get lower than Ga. Now the increase effect of the nuclear charge (81-49=32units) outweighs the effect of shielding due to the presence of the 4f as well as 5d electrons and so the $${\mathrm{\Delta }}_{i}H$$of Tl get higher than In.

Note:Shielding effect is defined as the effect in which the electrons in the inner side develop a sort of shield for the electrons in the outer shells due to which it does not let the appropriate charge towards the outermost electrons. The penetration effect is defined as the proximity of the electron in an orbital to the nucleus. On observing we get to know that the electron density for s orbitals is closer than p and d orbitals.