Question

Colour in transition metal compounds is attributed to:
A. Small size of metal ions
B. Absorption of light in the UV region
C. A complete $ns$ shell
D. An incomplete $\left( n-1 \right)d$ subshell

Answer 1

Hint: Recall the arrangement of electrons that is required for a compound to be coloured. Think about what might allow the compounds to be able to absorb different frequencies of light.

Complete step by step solution:
We will look at each of the options given step by step and determine which one of them is the correct answer.
- Option A: We know that the transition metals exist from the fifth period onwards and thus have a large number of shells that are occupied by electrons. The attribute of coloured compounds also cannot be related to the size of the metal ions. Thus, this is not the correct option.
- Option B: Transition metal atoms do not have any special attribute other than having their last electron in the d-subshell that allows them to absorb the UV rays.
- Option C: A complete $ns$ shell will not contribute to the coloured compounds as there is a requirement that a compound has to have unpaired electrons if it is coloured. Since the shells are filled, they cannot have unpaired electrons and thus, this cannot be the answer to the question.
- Option D: If a metal ion has an incomplete shell, this means that it has a lot of electrons that are unpaired. This indicates that the compounds that will be formed will be coloured since the electrons can jump to the empty subshells too.

Hence, the answer to this question is ‘D. an incomplete $\left( n-1 \right)d$ subshell’

Note: We know that UV rays are not visible to humans, so even if the transition metals could absorb these, they would not contribute to the coloured nature of their compounds.