Calculate the radius of the third orbit of $He^{+}$. (in $pm$)

When the electron of a hydrogen atom returns to the $L$ shell $(n=2)$ from shells of higher energy,we get a series of lines in the spectrum. This series is called:

Calculate the energy of half a mole of photons of a radiation with a frequency of $3 \times 10^{12} \ Hz$.

Explain the hydrogen spectrum using Bohr's model.

If the radius and energy of the second Bohr orbit of a hydrogen atom are $r_2$ and $E_2$, respectively, then the radius and energy of the third Bohr orbit will be, respectively:

Binding energy of a hydrogen atom is $13.6 \ eV$. The binding energy of a singly ionized helium atom is : ............ $eV$