An element of the third period has successive ionisation enthalpies (in $\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$) 577, 1816, 2744, 11577 and 14842. The atomic number of the element is

The very large jump from the third to the fourth ionisation enthalpy (2744 to 11577) shows that three electrons are valence electrons and the fourth comes from the inner core. A third-period element with three valence electrons is in group 13, i.e. Z = 13.