The successive ionisation enthalpies (in $\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$) of an element $\mathrm{X}$ are 738, 1451, 7733 and 10540. The most likely formula of its chloride is

There is a very large jump (1451 to 7733, about 5.3 times) after the second electron, showing that the third electron comes from an inner, noble-gas-like core. So X has two valence electrons, forms $\mathrm{X}{}^{2+}$ and the chloride $\mathrm{XCl}{}_2$.