The ratio of the radius of the $2^{nd}$ orbit of $H$ to the radius of the $3^{rd}$ orbit of $He^{+}$ is ......

The wavelength of a particle having an Einstein energy of $1.995 \times 10^{12} \ erg \ mol^{-1} \ photon^{-1}$ is .......... $\mathring{A}$.

$A$: An electron revolving around the nucleus does not fall into the nucleus.
$R$: An electron revolving around the nucleus is an orbital electron.

The ratio of ground state energy of $Li^{2+}$,$He^{+}$,and $H$ is:

How did Niels Bohr obtain detailed information regarding the structure of atoms and molecules?

What is the ratio of the velocity of an electron in the ground state of a hydrogen atom to the velocity of an electron in the second excited state of $He^{+}$?