Experimentally it is found that $12.8 \mathrm{eV}$ energy is required to separate a hydrogen atom into a proton and an electron. So the orbital radius of the electron in a hydrogen atom is $\frac{9}{x}\times {10}^{-10} \mathrm{m}$.

The value of the $x$ is: _____.

($1 \mathrm{eV}=1.6\times {10}^{-19} \mathrm{J}, \frac{1}{4\pi {\epsilon }_0}=9\times {10}^9 \frac{{\mathrm{Nm}}^2}{{\mathrm{C}}^2}$ and electronic charge $=1.6\times {10}^{-19} \mathrm{C}$)

Hence, the correct answer is 16.