If the mass number of an atom is $A = 40$ and its electron configuration is $1{s^2},\;2{s^2},\;2{p^6},\;3{s^2},\;3{p^6}$, the number of neutrons and protons in its nucleus will be

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 :

Describe the nuclear force and its characteristics. 

The nuclei of which one of the following pairs of nuclei are isotones

Assertion : Isobars are the elements having same mass number but different atomic number.
Reason : Neutrons and protons are present inside nucleus.