In the electronic configuration of which of the following elements is an electron arranged in the $5d$ orbital?

Which of the following $3d$ bivalent metal ions has the smallest number of unpaired $d$ electrons?

Which of the following pairs has similar ionic radii?

The electronic configuration of $Gd^{2+}$ is (at. no. of $Gd$ is $64$)

An atom with atomic number $21$ belongs to the category of

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.