The atoms of the same element having different masses but same chemical properties are called:

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: The nuclear density of nuclides ${ }_{5}^{10} B, { }_{3}^{6} Li, { }_{26}^{56} Fe, { }_{10}^{20} Ne$ and ${ }_{83}^{209} Bi$ can be arranged as $\rho_{ Bi }^{ N } > \rho_{ Fe }^{ N } > \rho_{ Ne }^{ N } > \rho_{ B }^{ N } > \rho_{ Li }^{ N }$.
Reason $R$: The radius $R$ of a nucleus is related to its mass number $A$ as $R = R_0 A^{1/3}$,where $R_0$ is a constant.
In the light of the above statements,choose the correct answer from the options given below:

The force between a proton and a proton in a nucleus is .......

The radius of a nucleus with atomic number $Z = 50$ is $9 \times 10^{-13} \ m$. The electric potential at its surface is:

The most stable particle in the Baryon group is

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