The ratio of the velocity of an electron in the ground state of a hydrogen atom to its velocity in the second excited state of a $He^{+}$ ion is:

When an electron revolves in a stationary orbit,then:

What is the frequency of the radiation emitted when an electron in a hydrogen atom undergoes a transition from $n = 3$ to the ground state?

Supposing the energy (in arbitrary units) of the energy levels in the hydrogen atom is given as under:

Energy level	$K$ $(n=1)$	$L$ $(n=2)$	$M$ $(n=3)$	$N$ $(n=4...n=\infty)$
Energy	$-864 \ a.u.$	$-216 \ a.u.$	$-96 \ a.u.$	$0 \ a.u.$

The excitation energy needed to raise the electron from $M$ level to $n = \infty$ would be:

The energy of an $H$ atom in the $n^{th}$ orbit is $E_n$. What will be the energy of the $n^{th}$ orbit of a singly ionized helium atom $(He^+)$?

What is the relation between radius, order of orbit, and nuclear charge for hydrogen-like species?