$\mathrm{A}_{92} \mathrm{U}^{238}$ nucleus decays to a ${ }_{82} \mathrm{~Pb}^{214}$ nucleus. The number of $\alpha$ and $\beta^{-}$particles emitted are

The ratio of the centripetal accelerations of the electron in two successive orbits of hydrogen is $81: 16$. Due to ${ }^3$ transition between these two states, the angular momentum of the electron changes by ( $h=$ Planck's constant)

The operation of a nuclear reactor is said to be critical when the value of neutron multiplication factor $K i s$

An $\alpha$-particle of energy $E$ is liberated during the decay of a nucleus of mass number 236. The total energy released in this process is