JEE Main · 2025 · Shift-IeasyPERI-143

The incorrect decreasing order of atomic radii is:

Classification of Elements & Periodicity · Class 11 · JEE Main Previous Year Question

Question

Options

a
$\text{Mg} > \text{Al} > \text{C} > \text{O}$
b
$\text{Al} > \text{B} > \text{N} > \text{F}$
c
$\text{Be} > \text{Mg} > \text{Al} > \text{Si}$
✓
d
$\text{Si} > \text{P} > \text{Cl} > \text{F}$

Correct Answerc

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

Detailed Solution

🧠 Atomic radius increases down a group — Be < Mg always The question asks for the incorrect decreasing order. In Group 2, Mg is below Be and has one more electron shell, so Mg is larger. Any order that puts Be above Mg in size is wrong.

🗺️ Check option (c) The sequence $\ce{Be > Mg > Al > Si}$ puts Be as larger than Mg. But Be is in Period 2 and Mg is in Period 3 of the same group. Mg must be larger. This order is wrong — it is the incorrect one.

The other options can be verified as correct trends (cross-period and cross-group comparisons work out).

$\boxed{\text{Answer: (c)}}$

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