The noble gas in the seventh period of the modern periodic table has the symbol.

An element with electronic configuration $\left[\mathrm{Kr}\right] 5{\mathrm{s}}^2 4{\mathrm{d}}^{10} 5{\mathrm{p}}^6 6{\mathrm{s}}^2 4{\mathrm{f}}^{14} 5{\mathrm{d}}^2$ belongs to

The successive ionization energies of an element $\mathrm{A}$ are ${\mathrm{IE}}_1=6.0 \mathrm{eV}, {\mathrm{IE}}_2=18.8 \mathrm{eV}, {\mathrm{IE}}_3=30.2 \mathrm{eV}$ and ${\mathrm{IE}}_4=120 \mathrm{eV}$ formula of oxide of element $\mathrm{A}$ is