The magnitude of kinetic energy $(K.E.)$ of an electron in a Bohr orbit is equal to which of the following?

The wavelength of a particular electron transition for $He^{+}$ is $100 \ nm$. The wavelength (in $\mathring{A}$) of $H$ atom for the same transition is

The ratio of velocity of electron in the second orbit of $He^{+}$ to the third orbit of $B^{4+}$ is $:-$

The energy of an electron revolving in the $n^{th}$ Bohr's orbit of an atom is given by the expression

If the radius of the electron orbit in the excited state of a hydrogen atom is $476.1 \ pm$, the energy of the electron in that excited state in $J$ is (Radius and energy of the electron in the first orbit of a hydrogen atom are $52.9 \ pm$ and $-2.18 \times 10^{-18} \ J$ respectively).

The wavelength of the first line of the Balmer series in the hydrogen spectrum is ................ $\mathring{A}$.