Why are the $E^{\circ}$ values for $Mn^{2+}$,$Ni^{2+}$,and $Zn^{2+}$ more negative than expected?

Which elements of the $3d$ series do not show variable valency?

Many transition metals form interstitial compounds. The characteristics of these interstitial compounds are:
$I$. They have high melting points,higher than those of pure metals.
$II$. They are very hard.
$III$. They retain metallic conductivity.
$IV$. They are chemically more reactive than the pure metals.

How many unpaired electrons are there in $Ni^{2+}$?

Which of the following is responsible for the stability of $Ce^{4+}$?

The number of $d$-electrons in $Cr^{3+}$ ion $(Z=24)$ is