Number of colourless lanthanoid ions among the following is __________.
Eu³⁺, Lu³⁺, Nd³⁺, La³⁺, Sm³⁺

Solution

Chemistry ma’am · 3 steps

Write the 4f count for each ion: an empty or full 4f shell has no f-f transition, so no colour.

1. Why colour. Lanthanoid ions are coloured because an electron jumps between partly filled 4f orbitals (an f-f transition). With 4f⁰ there is no electron to jump, and with 4f¹⁴ there is no empty f orbital to jump into, so both are colourless.
2. Get the 4f count. Remove three electrons from each atom. The 6s and 5d electrons go first, and then any remaining electrons come from 4f.

   Ion	Configuration	Colour
   Eu³⁺	[Xe] 4f⁶	pale pink
   Lu³⁺	[Xe] 4f¹⁴, full	colourless
   Nd³⁺	[Xe] 4f³	lilac
   La³⁺	[Xe] 4f⁰, empty	colourless
   Sm³⁺	[Xe] 4f⁵	yellow

3. So. Only La³⁺ and Lu³⁺ are colourless, so the answer is 2.

The common slip: 3. Treating Eu³⁺ as 4f⁷ like Gd³⁺ gives 3. It is Eu²⁺ that is 4f⁷, and Eu³⁺ is 4f⁶ and pale pink.

The idea to keep. For Ln³⁺ ions, find the 4f count first. 4f⁰ and 4f¹⁴ have no f-f transition, so they are colourless. 4f⁷ (Gd³⁺) is also colourless.

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