A nuclide at rest emits an $\alpha$-particle. In this process

The half-life period of a radioactive element $$x$$ is same as the mean life time of another radioactive element $$y$$. Initially, both of them have the same number of atoms. Then,

Polonium has a half-life of 140 days. If we take $$20 \mathrm{~g}$$ of polonium initially then the amount of it that remains after 280 days is

According to Bohr model of hydrogen atom, only those orbits are permissible which satisfy the condition