Question

3+ La is colorless but 4+ Lu is orange red why​

Answer 1

We have to give reason why La³⁺ is colourless while Lu⁴⁺ is orange red.

Solution : we know, colour of a metal ion depends on the unpaired electrons. If ion has no unpaired electrons it means, transition is not possible implies that ion doesn't absorb radiation in visible region hence it is colourless. But if ion has unpaired electrons, transition position and ion absorbs radiation in visible region so it is colorful.

Here La (Z = 57)

Electronic configuration of La = [Xe]5d¹6s²

so electronic configuration of La³⁺ = [Xe] 6s⁰ here it is clear that there is no any unpaired present in La³⁺ ion hence it is colourless.

Lu (Z = 71)

Electronic configuration of Lu = [Xe]4f¹⁴5d¹6s²

so electronic configuration of Lu⁴+ = [Xe]4f¹³ , you see 4f doesn't completely filled by electrons hence it has unpaired electrons so it is colorful and its colour is orange red.