JEE Chemistry 2025 Question with Solution

Given: We need to identify the incorrect decreasing order of atomic radii from the given options.

Find: Which option does not follow periodic trends.

Atomic radius generally decreases across a period from left to right and increases down a group.

For option C:

$$\text{Be} > \text{Mg} > \text{Al} > \text{Si}$$

But Be and Mg are in the same group, and atomic radius increases down the group, so:

$$\text{Mg} > \text{Be}$$

Also, within Period $3$:

$$\text{Mg} > \text{Al} > \text{Si}$$

Therefore, the correct decreasing order among these four elements should be:

$$\text{Mg} > \text{Be} > \text{Al} > \text{Si}$$

Hence,

$$\text{Be} > \text{Mg} > \text{Al} > \text{Si}$$

is the incorrect order.

Therefore, the correct option is C.

Given: The options list decreasing orders of atomic radii.

Find: The order that is incorrect.

Use these periodic trends:

1. Across a period, atomic radius decreases.
2. Down a group, atomic radius increases.

Check the decisive comparison in option C:

• Be is in Group $2$, Period $2$.
• Mg is in Group $2$, Period $3$.

Since Mg lies below Be in the same group:

$$\text{Mg} > \text{Be}$$

Now in Period $3$:

$$\text{Mg} > \text{Al} > \text{Si}$$

So the proper decreasing order should be:

$$\text{Mg} > \text{Be} > \text{Al} > \text{Si}$$

Thus the listed order

$$\text{Be} > \text{Mg} > \text{Al} > \text{Si}$$

reverses the first comparison and is therefore incorrect.

The solution contains contradictory text in one section, but the detailed working shows that the incorrect order is option C.

Therefore, the correct option is C.