To remove an inner, most tightly bound, electron from an atom of molybdenum requires energy of 20 keV. If this is to be done by allowing a photon to strike the atom, we have to find (a) the associate wavelength of the photon; (b) identify the region of the electromagnetic spectrum in which the photon lies; (c) and answer whether this process could be called a photoelectric effect.

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(a)

Wavelength corresponding to a photon that has energy of 20 keV required for making free the most tightly bound electron from a molybdenum atom can be found from the relation

(b)

As the wavelength of the photon required for removing the most tightly bound electron of a molybdenum atom is of the order of pm, it lies in the x-ray region of the electromagnetic spectrum.

(c)

In photoelectric effect a free electron is emitted from a metal surface by absorbing energy from a photon, the recoil momentum is shared by the lattice to which electron is weakly linked. On the other hand when a tightly bound electron is emitted from an atom, the recoil momentum is taken by the ionised atom.