Chemistry · Ch 8 — Transition and Inner Transition Elements

Colour and Spectra

8.12.3

Colour and Spectra

Some trivalent ions ( $\mathrm{M^{3+}}$ ) are coloured, in the solid state as well as in solution. The colour of a lanthanoid ion is due to f-f transitions, which correspond to energy in the visible region of the electromagnetic spectrum. The colour of ions having n f-electrons is about the same as those having (14 – n) f-electrons (where n is an integer 1-13):

Ln ion	No. of f-electrons	Colour
$\mathrm{Pr^{3+}}$	$\mathrm{4f^{\,2}}$	green	(14 - n) f-electrons = 14 - 2 = 12
$\mathrm{Tm^{3+}}$	$\mathrm{4f^{\,12}}$	green	n f-electrons = 12
$\mathrm{Nd^{3+}}$	$\mathrm{4f^{\,3}}$	pink	(14 - n) f-electrons = 14 - 3 = 11
$\mathrm{Er^{3+}}$	$\mathrm{4f^{\,11}}$	pink	n f-electrons = 11

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