more the charge on the atom, more is its stability.

electronic configuration of $\mathrm{Fe}^{2+}$ is $3 d^6$ while that of $\mathrm{Fe}^{3+}$ is $3 d^5$.

$\mathrm{Fe}^{2+}$ has a larger size than that of $\mathrm{Fe}^{3+}$.

$\mathrm{Fe}^{3+}$ ions are coloured.

$\mathrm{Cu}, \mathrm{Ni}, \mathrm{Zn}$

$\mathrm{Ni}, \mathrm{Au}, \mathrm{Cd}$

$\mathrm{Au}, \mathrm{Hg}, \mathrm{Pd}$

$\mathrm{Zn}, \mathrm{Cd}, \mathrm{Hg}$

$3 d^4 4 s^2 ; 3 d^{10} 4 s^1$

$3 d^5 4 s^1 ; 3 d^{10} 4 s^1$

$3 d^5 4 s^1 ; 3 d^9 4 s^2$

$3 d^4 4 s^2 ; 3 d^9 4 s^2$