The ratio of minimum wavelengths of Balmer and Paschen series of hydrogen atom will be

The activity of a radioactive sample is measured as No counts per minute at $$t=0$$ and $$\mathrm{N}_0 / \mathrm{e}$$ counts per minute at $$t=6 \mathrm{~min}$$. The time (in minutes) at which the activity reduces to half its value is.

The wavelength of $$k_\alpha$$-line characteristic $$X$$-rays emitted by an element is $$0.32 \mathop A\limits^o$$. The wavelength of the $$k_\beta$$-line emitted by the same element will be

A radioactive element $$X$$ convents into another stable element $$Y$$. Half-life of $$X$$ is 2 hrs. Initially only $$X$$ is present. After time $$t$$, the ratio of atoms of $$X$$ and $$Y$$ is found to be $$1: 4$$, then $$t$$ in hours is

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

The Rutherford scattering experiment proves that an atom consists of