Question

Explain why:
Transition elements form coloured compounds.

Answer 1

Hint: We know that Transition elements have unfilled d subshells. When transition elements form cations they influence the electron cloud. There are also elements with their d subshells incompletely filled and energy of all the d orbitals does not remain the same.

Complete step by step answer: We know that transition elements mostly form coloured compounds when they form a bond with nonmetals. Bonds involving transition elements are not ionic. Due to this whenever transition elements are positively charged they try to polarize anions. Transition element cations pull electron clouds towards them implying a formation of a covalent bond. Due to the polarization of anion compounds will be coloured. As anion size increases the covalent character of the bond increases.
When transition metals forming coordination compounds have incompletely filled d or f subshells they may have colour. In coordination compounds, different ligands split the degenerate orbitals differently as a result small energy difference arises between different d orbitals. Subshells d, f do not have similar type orbital shapes concerning axes as a result energy differences arise and splitting will be different for different ligands. Though f orbital energies vary this difference is small because f subshell is deeply embedded in the nucleus.
Whenever light falls on the transition element compounds electrons excite and electrons absorb energy and excite. When these electrons de-excite they release visible light wavelength. That’s why transition element compounds exhibit colour.

Note: The energy required to promote an electron in the s or p subshell is very high compared to that of promoting an electron in d or f subshell. S and p block elements do not contain empty subshells when they form compounds so the electron excitation requires very large energy so compounds of s and p block elements are mostly not coloured.