(a) (i) Nuclear fission & fusion: (ii) In nuclear fusion, the binding energy of the products is greater than the binding energy of reactants. In nuclear fission, binding energy of fragments is greater than the binding energy of the parent. This difference in binding energy is released in form of energy. OR (b) (i) Experimental determination of size of nucleus: Size of nucleus was determined experimentally by Rutherford by his alpha particle scattering experiment. In the experiment, a gold foil of thickness $2.1 \times 10^{-7}$ was bombarded by energetic $\alpha$-particles generated from ${}^{214}_{83}\mathrm{Bi}$ source.Scattered $\alpha$-particles were observed on a $\mathrm{ZnS}$ screen with the help of a microscope.It was observed that most of the $\alpha$-particles passed through the gold foil undeviated.About $14\% \alpha$-particles were scattered by an angle more than $1^{0}$.1 out of $8000 \alpha$-particles was scattered by an angle more than $90^{\circ}$.Very few $\alpha$-particles was scattered by an angle $180^{\circ}$.From these observations Rutherford calculated the impact parameter and distance of closest approach and concluded that the size of nucleus lies between $10^{-15} \text{ m}$ and $10^{-14} \text{ m}$.Relation between radius and mass number:$r = R_0 A^{1/3} \; (\text{ where } R_0=1.25 \text{ fm})$ (ii) Density of nucleus is independent of mass number: $\; \rho = \frac{M}{V}$ or, $\; \rho = \frac{m A}{\frac{4}{3} \pi r^3}$ or, $\; \rho = \frac{m A}{\frac{4}{3} \pi R_0^3 A} \; \; \text{ (since } r = R_0 A^{1/3} \text{ )}$ $\therefore \; \rho = \frac{m}{\frac{4}{3} \pi R_0^3}$ Hence, it is independent of mass number.