Chapter 23, Problem 23.1VP

Interpretation Introduction

To identify:

From the four highlighted metals in Figure P32.1, the cations of which of the two form coloured compound with ${Cl}^{-}$.

Answer to Problem 23.1VP

Solution:

Cations of chromium (green) and cobalt (yellow) form coloured compound with  ${Cl}^{-}$.

Explanation of Solution

The highlighted elements are calcium (red), chromium (green), cobalt (yellow), and zinc (blue).

Transition metal compounds are often colored because of partially filled d-orbitals. The d-orbitals are generally split into two groups of different energies. The difference in the energies of the two groups is designated $∆$. An electron from a lower energy d-orbital can absorb a photon of energy equal to $∆$ and move to be promoted to a higher energy d-orbital from the same subshell. If the energy of the photon corresponds to the visible region, the compound appears colored.

Calcium belongs to s-block element and does not have d-electrons. Hence,  it does not form coloured compounds.

Cations of chromium and cobalt have d-electrons which undergo d-d transition, and therefore, appear colored.

Zinc is also a d-block element but it has a completely filled 3d subshell. Therefore, a d-d transition is not possible and its compounds are colorless.

Conclusion

Transition elements having partially filled d-subshell form coloured compounds.