Which among the following elements has the highest first ionization enthalpy?

$X_{(g)} \to X^{+}_{(g)} + e^-$,$\Delta H = +720 \ kJ \ mol^{-1}$
Calculate the amount of energy required to convert $110 \ mg$ of $X$ atom in gaseous state into $X^{+}$ ion .................... $kJ$ (Atomic weight for $X = 7 \ g \ mol^{-1}$)

Which of the following has the minimum ionization energy?

The first ionisation energy of $Na$,$B$,$N$ and $O$ atoms follows the order

The element having the greatest difference between its first and second ionization energies is:

Of the following isoelectronic ions,the one which has the lowest ionisation potential is