\(\mathrm{Nd}^{+3}-[\mathrm{Xe}] 4 \mathrm{f}^3\)
\(\mathrm{Sm}^{+3}-[\mathrm{Xe}] 4 \mathrm{f}^5\)
\(\mathrm{Eu}^{+3}-[\mathrm{Xe}] 4 \mathrm{f}^6\)
\(\mathrm{Lu}^{+3}-[\mathrm{Xe}] 4 \mathrm{f}^{14}\)
\(\mathrm{La}^{+3}\) and \(\mathrm{Lu}^{+3}\) do not show any colour because no unpaired electron is present.
[પરમામાણ્વીય ક્રમાંક ${Gd}=64$ ]