Question

Photoelectric effect is maximum in:
(a) Cs
(b) Na
(c) K
(d) Li

Answer 1

Hint: The emission of electrons from a material, when it is exposed to electromagnetic radiation is called photoelectric effect. Such emitted electrons are called photoelectrons. It is maximum in a metal which belongs to group 1 and has the largest ionic radius among the elements of its group.

Complete step by step solution:
We know that ionisation potential is the energy required to remove an electron from the outermost valence shell. For the emission of electrons, the ionisation potential of the atom, ion or molecule should be very low. Then only electrons can be emitted easily. All the elements given in the options belong to the alkali metal group. We know that as we move down the group. The ionisation potential decreases. Caesium is the last element in the alkali metal group. Therefore, the ionisation potential of Caesium is less compared to the other metals and when it is subjected to electromagnetic radiation, the electrons are emitted or ejected easily than other metals in the group.
Hence, photoelectric effect is maximum in Cs. The correct answer to this question is option (a).

Additional Information:
-Photoelectric effect can also be explained as the transfer of light energy to the electrons, which helps them to escape from the valence shell of the particular atom. These emitted electrons can conduct electricity.
-Any alteration in the intensity of light can cause changes in the kinetic energy of the electrons which are emitted from the metal.
-So, according to the theory, a dim light shows a time lag between initial shining of its light and the subsequent emission of an electron.
-The material on which the electromagnetic radiation is exposed is called the anode. Thus, it is called a photoelectrode.

Note: We need to keep in mind that for electrons to easily be removed from a valence shell, the atomic radius should be more, so that the attraction between the valence electron and nucleus is less. So, electrons can be easily emitted by light.