The watermelon model of the atom was proposed by:

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

$A$ certain electronic transition from an excited state to the ground state of the $H$ 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?

If an electron falls from $n = 3$ to $n = 2$,then the emitted energy is $..........$ $eV$.

The figure that is not a direct manifestation of the quantum nature of atoms is:

$(i)$ The energy associated with the first orbit in the hydrogen atom is $-2.18 \times 10^{-18} \, J \, atom^{-1}$. What is the energy associated with the fifth orbit?
$(ii)$ Calculate the radius of Bohr's fifth orbit for the hydrogen atom.