Chemistry Q59 – JEE-MAIN 2025

Home Chemistry Periodicity & ElementsQuestion #539

Choose the incorrect trend in the atomic radii (r) of the elements.

Clue 1 of 3: General Periodic Trends

Recall how atomic radius changes across a period and down a group in the periodic table.

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Ninja Strategy•Apply General Periodic Trends

Quickly recall that atomic radius decreases across a period and increases down a group. Option 2 ( $r_{M g} < r_{A l}$ ) incorrectly suggests that Aluminum (further right) is larger than Magnesium (further left) in the same period.

🧠 Full Solution Path

Step 1: Recall Periodic Trends for Atomic Radius✦ Active

Atomic radius generally decreases from left to right across a period due to an increase in effective nuclear charge, which pulls the electrons closer to the nucleus. Atomic radius generally increases from top to bottom down a group due to the addition of new electron shells, which are further from the nucleus.

Step 2: Evaluate Each Option based on Trends○ Expand

1. $r_{A t} < r_{C s}$ : Cesium (Cs) is in Group 1, Period 6. Astatine (At) is in Group 17, Period 6. Since atomic radius decreases across a period, $r_{C s} > r_{A t}$ . So, $r_{A t} < r_{C s}$ is correct.

2. $r_{M g} < r_{A l}$ : Magnesium (Mg) is in Group 2, Period 3. Aluminum (Al) is in Group 13, Period 3. Since atomic radius decreases across a period, $r_{M g} > r_{A l}$ . Therefore, $r_{M g} < r_{A l}$ is incorrect.

3. $r_{R b} < r_{C s}$ : Rubidium (Rb) is in Group 1, Period 5. Cesium (Cs) is in Group 1, Period 6. Since atomic radius increases down a group, $r_{C s} > r_{R b}$ . So, $r_{R b} < r_{C s}$ is correct.

4. $r_{B r} < r_{K}$ : Potassium (K) is in Group 1, Period 4. Bromine (Br) is in Group 17, Period 4. Since atomic radius decreases across a period, $r_{K} > r_{B r}$ . So, $r_{B r} < r_{K}$ is correct.

💡 Teacher's Secret Hint

Pay close attention to elements within the same period versus the same group.

Step 3: Identify the Incorrect Trend○ Expand

Based on the evaluation, the trend $r_{M g} < r_{A l}$ is incorrect because Magnesium is to the left of Aluminum in the same period, and atomic radius decreases across a period.

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Chemistry Question 59 – JEE-MAIN 2025

Clue 1 of 3: General Periodic Trends

🧠 Full Solution Path