A radioactive sample has half-life of 3 years. The time required for the activity of the sample to reduce to $$\frac{1}{5}$$th of its initial value is about

In the following equation representing $$\beta^{-}$$ decay, the number of neutrons in the nucleus $$X$$ is

$${ }_{83}^{210} \mathrm{Bi} \longrightarrow X+e^{-1}+\bar{v}$$

A nucleus with mass number 220 initially at rest emits an alpha particle. If the $$Q$$ value of reaction is $$5.5 ~\mathrm{MeV}$$. Calculate the value of kinetic energy of alpha particle

The radius of hydrogen atom in the ground state is 0.53$$\mathop A\limits^o $$. After collision with an electron, it is found to have a radius of 2.12$$\mathop A\limits^o $$, the principal quantum number $$n$$ of the final state of the atom is