Question

Aluminium exists in $+3$ stable state whereas thallium exists in both $+1$ & $+3$ oxidation state. The reason for this is

• A. Inert pair effect
• B. Lanthanoid contraction
• C. Diagonal relationship
• D. None of these

Answer 1

The correct option is A

Inert pair effect

Explanation for correct option:

Inert pair effect:

1. Inert pair effect is the tendency of two electrons in the outermost s-orbital to remain unshared in compounds of p-block elements.
2. Thallium exists in $+1$oxidation state due to the inert pair effect.
3. Aluminium does not exist in $+1$oxidation state as it does not show inert pair effect due to the small size and absence of d and f-orbitals.

Explanation for incorrect options:

B Lanthanoid contraction

• Lanthanoid contraction is the steady decrease in the size of lanthanoid ions with the increase in atomic number.
• So, this concept is not applicable here.

C Diagonal relationship

• Diagonal relationship exists between elements placed diagonally to each other and aluminium and thallium are elements of the same group.
• So the option is not applicable

D None of these

This option is also not applicable as it is understood from above that option A is applicable.

Hence, option A is correct, Aluminium exists in $+3$ stable whereas thallium exists in both $+1$ & $+3$ oxidation state due to inert pair effect.