d- and f-Block Elements

The correct order of ionic radii of Y3+, La3+, Eu3+ and Lu3+ is (Atomic nos. Y = 39, La = 57, Eu = 63, Lu = 71)

1. Y3+ < La3+ < Eu3+ < Lu3+
2. Y3+ < Lu3+ < Eu3+ < La3+
3. Lu3+ < Eu3+ < La3+ < Y3+
4. La3+ < Eu3+ < Lu3+ < Y3+

Answer

BONUS

Solution

Y (Z = 39) lies just above La in group 3, and in the lanthanoid series the M3+ radii decrease from La3+ to Lu3+ (lanthanoid contraction, due to poor shielding by 4f electrons), so La3+ > Eu3+ > Lu3+.

The lanthanoid contraction is so large that Lu3+ becomes even smaller than Y3+. The measured ionic radii (coordination number 6) are La3+ 103 pm, Eu3+ 95 pm, Y3+ 90 pm and Lu3+ 86 pm.

∴ Correct order: Lu3+ < Y3+ < Eu3+ < La3+, which matches none of the four options.

Option (2) assumes Y3+ is the smallest because it has one shell fewer, but this ignores the lanthanoid contraction; (1), (3) and (4) are wrong orders as well.

Answer: Bonus

Note: The official answer key gives option (2). Our faculty team has re-checked this question and finds that no option is fully correct, so this question should be dropped (bonus: full marks to everyone who attempted it).