JEE Chemistry 2025 Question with Solution

Given: We must choose the incorrect trend in atomic radii.

Find: Which relation does not follow periodic trends.

Atomic radius generally decreases across a period from left to right because effective nuclear charge increases. Atomic radius generally increases down a group because additional electron shells are added.

Now evaluate each option:

1. $r_{Br} < r_K$ Br and K are in the same period, and K lies to the left of Br. So K has larger atomic radius. This trend is correct.

2. $r_{Mg} < r_{Al}$ Mg and Al are in the same period, and atomic radius decreases from left to right.

$$r_{Mg} > r_{Al}$$

So the given relation is incorrect.

3. $r_{Rb} < r_{Cs}$ Rb and Cs are in the same group, and atomic radius increases down the group. So this trend is correct.

4. $r_{At} < r_{Cs}$ Cs and At are in the same period, and atomic radius decreases from left to right. Hence Cs is larger than At. This trend is correct.

Therefore, the incorrect trend is $r_{Mg} < r_{Al}$. The correct option is B.

Given: Four comparisons of atomic radii are provided.

Find: The option that contradicts periodic trends.

Use these rules:

• Across a period: atomic radius decreases from left to right.
• Down a group: atomic radius increases from top to bottom.

For option A:

$$r_{Br} < r_K$$

Both are in Period $4$. Since K is farther left, K is larger. So option A is correct.

For option B:

$$r_{Mg} < r_{Al}$$

Both are in Period $3$. Mg is to the left of Al, so Mg should be larger.

$$r_{Mg} > r_{Al}$$

Hence the stated trend is wrong.

For option C:

$$r_{Rb} < r_{Cs}$$

Both are in Group $1$. Radius increases down the group, so Cs is larger. This is correct.

For option D:

$$r_{At} < r_{Cs}$$

Both are in Period $6$. Cs is on the left side and At is on the right side, so Cs is larger. This is correct.

Thus only option B is incorrect.