$\mathrm{O}_2, \mathrm{H}_2 \mathrm{O}, \mathrm{Cr}(\mathrm{OH})_3$

$\mathrm{NO}_2, \mathrm{H}_2 \mathrm{O}, \mathrm{Cr}_2 \mathrm{O}_3$

$\mathrm{N}_2, \mathrm{Cr}_2 \mathrm{O}_3, \mathrm{H}_2 \mathrm{O}$

$\mathrm{N}_2 \mathrm{O}, \mathrm{Cr}(\mathrm{OH})_3$

$\mathrm{Ce}, \mathrm{Pr}$

Eu, Gd

Eu, Yb

Lu, Er

$\mathrm{Cr}, \mathrm{Mo}$ and W have high melting points.

With increase in number of unpaired. electrons melting point increases.

$\mathrm{Mn}^{3+}$ is more stable than $\mathrm{Mn}^{2+}$.

They show variable oxidation states.