The incorrect decreasing order of atomic radii is

Solution

Chemistry ma’am · 3 steps

Use two trends: radius shrinks across a period and grows down a group. Find the order that breaks one of them.

1. Across a period. From left to right in a period, nuclear charge rises but the electrons go into the same shell. The pull gets stronger, so the atomic radius decreases.
2. Down a group. Going down a group, each element adds a new shell. The outer electrons sit farther from the nucleus, so the atomic radius increases. An atom in period 3 is usually larger than an atom in period 2.

   Option	Order given	Check against the trends	Correct?
   (a)	Si>P>Cl>F	Si, P, Cl lie left to right in period 3. F is in period 2 and above Cl.	Yes
   (b)	Mg>Al>C>O	Mg, Al are in period 3. C, O are in period 2, with C left of O.	Yes
   (c)	Al>B>N>F	Al is below B in group 13. B, N, F lie left to right in period 2.	Yes
   (d)	Be>Mg>Al>Si	Be is in period 2, above Mg in group 2, so Be is smaller than Mg.	No

3. So. Mg, Al, Si do decrease across period 3, but Be cannot come first. The correct order is Mg>Al>Si>Be. The incorrect order is option (d).

The idea to keep. For radius questions, first place each element by period and group. A new shell down a group outweighs everything else, so a period 2 atom is almost always smaller than its period 3 neighbours.

Answer from the NTA final answer key. Solution by PrepWiser.