JEE Chemistry 2025 Question with Solution

Given: The question asks for the elements of Group 13 having the highest and lowest first ionisation enthalpies.

Find: The correct pair among the given options.

First ionisation enthalpy generally decreases down a group because atomic size increases and shielding also increases. In Group 13, Boron (B) has the smallest size, so it has the highest first ionisation enthalpy.

For the lower end of the group, the solution states that due to the poor shielding effect of the inner d-electrons, Gallium (Ga) is identified as the element with the lowest first ionisation enthalpy among the provided options.

Therefore, the required pair is Boron (B) and Gallium (Ga).

The correct option is A.

Given: Elements of Group 13 are B, Al, Ga, In, Tl.

Find: Which one has the highest and which one has the lowest first ionisation enthalpy.

Ionisation enthalpy is the energy required to remove the outermost electron from an isolated gaseous atom.

The usual group trend is that ionisation enthalpy decreases on moving down the group because:

1. Atomic size increases.
2. The valence electron is farther from the nucleus.
3. Shielding effect increases.

So, the element at the top of the group, Boron (B), has the highest first ionisation enthalpy.

The provided solution further notes an exception-type effect due to poor shielding by d and f electrons in heavier elements. Based on the extracted solution, it concludes that Gallium (Ga) is the lowest among the provided options.

Hence, the pair is B & Ga.

So, the correct option is A.