The space between the proton and the electron in a hydrogen atom is:

The ratio of the radius of the first excited state of $Li^{2+}$ to the radius of the second excited state of $H$ is ....

What is the wavenumber of the photon emitted during transition from the orbit $n=5$ to that of $n=2$ in hydrogen atom (in $cm^{-1}$)?
$\left[R_{H}=109677 \ cm^{-1}\right]$

The energy of a photon is $3 \times 10^{-12} \ erg$. What is its wavelength in $nm$? $(h = 6.62 \times 10^{-27} \ erg \cdot s; \ c = 3 \times 10^{10} \ cm/s)$

Write down the postulates of Bohr's model for the hydrogen atom.

Circumference of the first orbit of hydrogen atom is given by the formula :