Ratio of specific charge of $\alpha$-particle to that of proton is

Two protons exert a nuclear force on each other. The distance between them is approximately:

Consider the following statements:
$I$. All isotopes of an element have the same number of neutrons.
$II$. Only one isotope of an element can be stable and non-radioactive.
$III$. All elements have isotopes.
$IV$. All isotopes of carbon can form chemical compounds with oxygen-$16$.
Choose the correct option regarding an isotope.

The electric potential at the surface of an atomic nucleus $(Z=50)$ of radius $9 \times 10^{-13} \text{ cm}$ is . . . . . . $\times 10^6 \text{ V}$.

The radius $R$ of a nucleus of mass number $A$ can be estimated by the formula $R = (1.3 \times 10^{-15}) A^{1/3} \; m$. It follows that the mass density of a nucleus is of the order of $(M_{\text{prot}} \cong M_{\text{neut}} = 1.67 \times 10^{-27} \; kg)$.

Assume that protons and neutrons have equal masses. The mass of a nucleon is $1.6 \times 10^{-27} \ kg$ and the radius of a nucleus is $R = 1.5 \times 10^{-15} A^{1/3} \ m$. The approximate ratio of the nuclear density to the water density is $n \times 10^{13}$. The value of $n$ is $.................$