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

Solution

Chemistry ma’am · 3 steps

Across a period atoms shrink, so Mg, which comes before Al, must be the bigger one.

1. Two trends. Down a group, each element adds a new shell, so the radius increases. Across a period, the shell stays the same but the nuclear charge rises, so the electrons are pulled in and the radius decreases.
2. Locate each pair. First check whether the two elements share a group or a period. Then apply the matching trend and compare it with the inequality given.

   Option	Relation	Actual radii (pm)	Trend holds?
   (a) r_(Rb) < r_(Cs)	Group 1, Cs is lower	Rb 244, Cs 262	Correct
   (b) r_(At) < r_(Cs)	Period 6, At is far to the right	At 140, Cs 262	Correct
   (c) r_(Br) < r_K	Period 4, Br is far to the right	Br 114, K 231	Correct
   (d) r_(Mg) < r_(Al)	Period 3, Al is to the right of Mg	Mg 160, Al 143	Incorrect

3. So. Al has one more proton pulling on the same third shell, so r_(Al) < r_(Mg). The trend in (d) is reversed, so (d) is the incorrect one.

The idea to keep. First ask: same group or same period? Down a group, add shells and the atom grows. Across a period, add protons to the same shell and the atom shrinks.

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