Experimentally it is found that 12.8 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 $\_\_\_\_$ . $\left(1 \mathrm{eV}=1.6 \times 10^{-19} \mathrm{~J}, \frac{1}{4 \pi \epsilon_0} 9 \times 10^9 \mathrm{Nm}^2 / \mathrm{C}^2\right.$ and electronic charge $\left.=1.6 \times 10^{-19} \mathrm{JC}\right)$