The first use of quantum theory to explain the structure of the atom was made by

The energy of an electron in the first Bohr orbit of the hydrogen atom is $-13.6 \ eV$. Calculate the energy of an electron in the first Bohr orbit of $He^{+}$ in $eV$.

What is the ratio of the wave number of the first line (lowest energy line) of the Balmer series of the $H$ atomic spectrum to the first line of its Brackett series?

The energy of an electron in a hydrogen atom in its ground state is $-13.6 \ eV$. The energy of the level corresponding to the quantum number equal to $5$ is ............. $eV$.

The frequency and energy of a photon with a wavelength of $4000 \ \mathring{A}$ are respectively.......

The postulate of Bohr theory that electrons jump from one orbit to the other,rather than flow,is according to: