The higher oxidation states of transition elements are found to be in combination with $A$ and $B$,which are:

The elements belonging to the first transition series have atomic numbers from

The maximum number of unpaired electrons is present in

Which transition elements of the $3d$ series have an exceptional electronic configuration?

Most transition metals exhibit several characteristic properties. Which of the following statements are correct?
$I$: Form sets of compounds which display different oxidation states of the metal.
$II$: Form coloured ions in solution.
$III$: Burn vigorously in oxygen.
$IV$: Act as catalysts.

$Zr$ $(Z=40)$ and $Hf$ $(Z=72)$ have similar atomic and ionic radii because of :