Two nucleons are at a separation of $1 \times {10^{ – 15}}m$. The net force between them is ${F_1}$, if both are neutrons, ${F_2}$ if both are protons and ${F_3}$ if one is a proton and other is a neutron. In such a case

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

How is an atom represented ? Why?

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 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 statement, choose the correct answer from the options given below :