Assertion (A): Aluminium is more electropositive than Thallium because E°(Al³⁺/Al) is more negative, whereas E°(Tl³⁺/Tl) is positive.

Reason (R): The sum of the first three ionization enthalpies of Boron is much higher than that of Aluminium. Hence, Boron forms covalent compounds, while Aluminium forms Al³⁺.

Choose the correct option:

• (1) Both Assertion and Reason are true, and Reason is the correct explanation.
• (2) Both Assertion and Reason are true, but Reason is not the correct explanation.
• (3) Assertion is true, but Reason is false.
• (4) Assertion is false, but Reason is true.

Let us examine the Assertion first.

The standard reduction potential of aluminium:

Al³⁺ + 3e^- → Al

has a highly negative value. This indicates that aluminium readily loses electrons and behaves as a strongly electropositive metal.

On the other hand, thallium shows a positive reduction potential for the Tl³⁺/Tl system. Due to the inert pair effect, thallium prefers the +1 oxidation state rather than +3. Therefore, thallium is less electropositive compared to aluminium.

Hence, the Assertion is correct.

Now consider the Reason.

Boron has a very small atomic size and high ionization enthalpy. Removal of three electrons from boron requires a very large amount of energy. As a result, boron generally forms covalent compounds rather than B³⁺ ions.

Aluminium has comparatively lower ionization enthalpy and can form Al³⁺ ions more easily. Therefore, the Reason is also correct.

However, the Reason discusses the difference between Boron and Aluminium, whereas the Assertion compares Aluminium and Thallium. Therefore, the Reason does not explain the Assertion.

Final Answer

Option (2) – Both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.