Atomic masses of two oxygen isotopes ${ }_8^{16} \mathrm{O}$ and ${ }_8^{18} \mathrm{O}$ are 15.99491 u and 17.99916 u , respectively, where $u$ is the atomic mass unit. Masses of protom and neutron are given by 1.00727 u and I .00866 u , respectively. The speed of light is c . What is the difference between the binding energies of ${ }_8^{18} \mathrm{O}{ }_8^{16} \mathrm{O}$ nuclei in units of $u c^2$ ?

Consider the Bohr model of an atom where an electron of charge $-e$ revolves around a nucleus of charge $+e$ in an orbit of radius $r$. The electron has an orbital angular momentum $2 h$. If the nucleus had charge $+2 e$, what would have been the radius of the orbit of the electron with the same principal quantum number?

Consider that in a fission of a single Plutonium ( $\mathrm{Pu}^{239}$ ) atom 207 MeV energy is released. Assuming all atoms of Plutonium undergo fission, which of the following options is the closest estimate of the amount of Plutonium required for a 20,000 units of TNT explosion? (1 unit of TNT $=4.184 \times 10^9 \mathrm{~J}, 1 \mathrm{eV}=1.6 \times 10^{-19} \mathrm{~J}$ )