If the binding energy of electrons in a metal is $250 \ kJ \ mol^{-1}$,what should be the threshold frequency of the striking photons in order to free an electron from the metal surface?

The ionisation energy of a certain element is $1540 \ kJ \ mol^{-1}$. When the atoms of this element are in the first excited state,the ionisation energy is only $300 \ kJ \ mol^{-1}$. Determine the region of the electromagnetic wave spectrum in which the light is emitted during a transition from the first excited state to the ground state.

If an electron is moving in the $n^{th}$ orbit around the nucleus with a velocity $v$ and radius $r$,then the value of $n$ is .........

The wave number of the first line of the Balmer series in the hydrogen spectrum is $15200 \, cm^{-1}$. What will be the wave number of the first line of the Balmer series for the $Li^{2+}$ ion in $cm^{-1}$?

The $Bohr$ model of an atom could not account for:

$A$ bulb emits light of wavelength $4500 \ \mathring{A}$. The bulb is rated at $150 \ \text{W}$ and $8\%$ of the energy is emitted as light. How many photons are emitted by the bulb per second?