The electric potential at the surface of an atomic nucleus (Z=50) of radius 9.0 x 10^-15 m is : 

8 x 10⁶ V

$\text{Number of proton inside the nucleus is Z= 50 , i.e total charge inside the nucleus is } Q = 50 \times 1.6 \times 10^{-19}$

$\Rightarrow V = \frac{kQ}{R} = 9\times 10^9 \times \frac{50\times 1.6\times 10^{-19}}{9\times 10^{-15}} = 8 \times 10^6V$