The successive ionisation enthalpies of an element $X$ in $\left(\mathrm{kJ} \mathrm{mol}^{-1}\right)$ are 1012, 1907, $2955,4955,6275$ and 21,260 respectively, the element $X$ is

\begin{array}{ll} \hline First ionisation enthalpy & 1012 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \hline Second ionisation enthalpy & 1907 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \hline Third ionisation enthalpy & 2955 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \hline Fourth ionisation enthalpy & 4955 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \hline Fifth ionisation enthalpy & 6275 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \hline Sixth ionisation enthalpy & 21,260 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \hline \end{array} Since, sixth ionisation enthalpy is much higher, it means that the removal of sixth electron is from the stable configuration, therefore the element is phosphorus $(\mathrm{P})$ having 5 electrons in the valence shell.