Calculate the radius of the first orbit of $He^{+}$. (in $pm$)

$A$ certain atomic transition from an excited state to the ground state of the hydrogen atom in one or more steps gives rise to four lines in the ultraviolet region of the spectrum. How many lines does this transition produce in the infrared region of the spectrum?

In the Bohr series of lines of the hydrogen spectrum,the third line from the red end corresponds to which one of the following inter-orbit jumps of the electron for Bohr orbits in an atom of hydrogen?

Bohr’s atomic theory gave the idea of :

Which formula is used to calculate the energy of a species containing $1$ electron?

If the radius of the hydrogen atom is $53 \, pm$, the radius of the $He^{+}$ ion is closest to $...... \, pm$.