If the radius of the first Bohr orbit is $r$, then the radius of the second Bohr orbit will be

The possible quantum numbers for a $3d$ electron are:

In the hydrogen atom, the electron makes a transition from the higher orbit $(i)$ to a lower orbit $(f)$. The ratio of the radius of the orbits is given by $r_i : r_f = 16 : 4$. The wavelength of the photon emitted due to this transition is . . . . . . nm. (Given Rydberg constant $R = 1.0973 \times 10^7 \text{ m}^{-1}$)

In a hydrogen atom,which quantity is an integral multiple of $\frac{h}{2\pi}$?

The angular momentum of an electron in the $2^{nd}$ excited state of a Helium ion $(He^+)$ is

$(a)$ Using the Bohr model,calculate the speed of the electron in a hydrogen atom in the $n = 1, 2,$ and $3$ levels.
$(b)$ Calculate the orbital period in each of these levels.