The energy of an electron in the $n^{th}$ orbit of a hydrogen atom is given by:

If an electron falls from $n = 3$ to $n = 2$,then the emitted energy is $..........$ $eV$.

If the wavelength of the first line of the Paschen series of a hydrogen atom is $720 \ nm$,then the wavelength of the second line of this series is $.......... \ nm$. (Nearest integer)

In the Bohr model of the atom,when an electron undergoes a transition from $n = 1$ to $n = 3$,the energy absorbed or emitted is ........

The spectrum of helium is similar to which of the following?

The radius of the first Bohr's orbit of the hydrogen atom is ............. $\mathring{A}$.