Formula of nickel oxide with metal deficiency defect in its crystal is ${\mathrm{Ni}}_{0.98}\mathrm{O}$. The crystal contains ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Ni}}^{3+}$ ions. The fraction of nickel existing as ${\mathrm{Ni}}^{2+}$ ions in the crystal is