0Inert Pair Effect in Group 13
In the modern periodic table, the elements of the boron family present in group 13 are:
Boron (B), Aluminium (Al), Gallium (Ga), Indium (In) and Thallium (Tl)
Their general electronic configuration of the valence shell is ns 2 np 1. They have a total of 3 electrons in their valence shell. Thus, the elements of the boron
family adopt the oxidation states of +3 (group oxidation state) and +1. The +3 oxidations state is favourable for the lighter elements of the group,
whereas, +1 oxidation state is preferable for the heavier elements of the group; for example, Tl has the stable oxidation state of +1.
The stability of the oxidation state two units less than the group oxidation state of the heavier members can be explained by the concept called the inert pair effect. To explain this let’s look into the electronic configuration of each member of the boron family:
B Al Ga In Tl
B
Al
Ga
In
Tl
[He]2s22p1
[Ne]3s23p1
[Ar]3d104s24p1
[Kr]4d105s25p1
[Xe]4f145d106s26p1
The elements present in periods 4, 5 and 6 have d-orbital (f- orbital for 6 th period) electrons along with s- and p-orbitals with electrons. The
shielding effect of different orbitals follows the order:
s > p > d > f
Due to the poor shielding effect of the intervening d and f-orbitals, the effective nuclear charge increases which tightly holds the ns electrons. As
a result, the inert pair of ns electrons are more tightly held by the nucleus, and hence, they can not participate in bonding. The remaining np
electron (only 1) involves in bonding. Thus, +1 oxidation state predominates in Tl. The reluctance of ns- electrons to take part in bonding due to
the poor shielding effect of d and f- orbitals on moving down a group is called inert pair effect.
