Discuss the pattern of variation in the oxidation states of B to Tl.

Explanation

1. Since B and Al have no d -or f-electrons, they do not exhibit inert pair effect. Therefore, they exhibit only one oxidation state of +3 due to presence of two electrons in the s -and one electron in the p-orbital of the outermost shell.
2. The remaining elements from Ga to Tl contain d or both d and J electrons in their inner shells and hence exhibit +3 as well as + 1 oxidation states due to inert pair effect.
3. As the number of d -and J-electrons increases on moving down the group, the inert pair effect becomes more and more prominent. As a consequence, the stability of+ 1 oxidation state increases (i.e., Ga < In < Tl), while that of +3 oxidation state decreases (i.e., Ga > In > TI). In fact, the + 1 oxidation state of Tl is more stable than its +3 oxidation state.