The electrons which take part in order to exhibit variable oxidation states by transition metals are

Pairs of elements with the same number of electrons in their respective $4f$ orbital are [Atomic number: $Eu-63, Gd-64, Dy-66, Ho-67, Tm-69, Yb-70, Lu-71, Hf-72$]. Choose the correct answer from the options given below:

Elements $A$ and $B$ with their respective electronic configurations $3d^{10} 4s^1$ and $4d^{10} 5s^1$ in their outermost shell are

The $CORRECT$ ionisation energy order is

Why are transition elements hard and do they possess high melting and boiling points?

The $I.P.$ values of $Cr, Zn, Mn$,and $Hg$ are $7.4 \ eV, 6.7 \ eV, 10.4 \ eV$,and $9.3 \ eV$. These $I.P.$ values are not listed in the order of the elements. Identify the correct $I.P.$ value for the given elements.