JEE Chemistry 2025 Question with Solution

Given: The elements are In, Tl, Al, Pb, and Ge.

Find: The most stable oxidation states of the elements with the highest and lowest first ionisation enthalpies, respectively.

From the solution working, we compare the positions of the given elements in the periodic table and use the trend that first ionisation enthalpy generally increases across a period and decreases down a group.

The extracted working identifies Germanium (Ge) as the element with the highest first ionisation enthalpy and Thallium (Tl) as the element with the lowest first ionisation enthalpy.

For oxidation states:

• Germanium (Ge), a Group $14$ element, has the most stable oxidation state $+4$.
• Thallium (Tl), a Group $13$ element, shows the inert pair effect, so its most stable oxidation state is $+1$.

Therefore, in the order highest and lowest first ionisation enthalpies, the required oxidation states are $+4$ and $+1$. The correct option is D.

There is a discrepancy in the answer key and one part of the solution, but the final worked conclusion in the detailed solution supports $+4$ and $+1$.

Given: Elements In, Tl, Al, Pb, and Ge are to be compared.

Find: Stable oxidation states of the elements having the highest and lowest first ionisation enthalpies.

Identify the trend: First ionisation enthalpy generally decreases down a group and increases across a period. For heavier p-block elements, the inert pair effect also affects the stability of oxidation states.

Apply to the given elements:

• Ge is higher and relatively smaller among the listed heavier p-block elements, so it has the highest first ionisation enthalpy in the accepted solution.
• Tl lies lower in Group $13$ and has the lowest first ionisation enthalpy among the given elements.

Determine stable oxidation states:

• Ge most commonly shows $+4$.
• Tl most stably shows $+1$ because the $ns^2$ pair remains less available for bonding.

Conclude: The required pair is $+4$ and $+1$. Hence, the correct option is D.