Which of the following has second ionisation potential less than expected?

If an atom has electronic configuration $1s^2 2s^2 2p^6 3s^2 3p^6 3d^3 4s^2$,it will be placed in :-

Which of the following electronic configurations of a transition metal atom shows the highest oxidation state?

The increase in the atomic radii of the third $(5d)$ series of transition elements is very small,which may be accounted for by the filling of '$X$' orbitals before '$Y$' orbitals. $X$ and $Y$ are:

What may be the stable oxidation states of the transition elements with the following $d$ electron configurations in the ground state of their atoms: $3d^3$,$3d^5$,$3d^8$,and $3d^4$?

Which of the following statements about the interstitial compounds is incorrect?