Question

Which alkali metal does not exhibit photoelectric effect _____

Answer 1

Alkali metals are the group one elements consisting of hydrogen (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), Cesium (Cs), and Francium (Fr). These elements have unpaired electrons in their valence shells. If energy is provided to any element then it gets excited and leads to the emission of the valence electron.

Complete answer:
Alkali metals are s-block elements having one unpaired electron in their $ns$ orbital. Due to the presence of this one orbit having one electron extra, most of the alkali metal exists as mono positive ions in their ground state. They attain this state because they have only one electron more in order to get the nearest noble gas configuration.
The photoelectric effect was discovered by Albert Einstein $1887$ and was awarded the Nobel prize for the same in the year of $1921$ . This effect is defined as “the removal of valence shell electrons when a ray/light of suitable frequency and wavelength falls on it.”
However, for the removal of valence electrons from the valence shell, the minimum amount of energy is required. The electron will only be freed from the orbit if and only if that minimum energy is provided. The minimum energy required for the removal of valence shell electrons is known as ionization energy. It is termed ionization energy because it converts an atom to an ion.
Out of all the alkali metals hydrogen (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), Cesium (Cs), and Francium (Fr), only lithium does not show any sort of photoelectric effect due to its high value of ionization energy.

Note:
The minimum energy to remove an electron is necessary to condition for the photoelectric effect. If this minimum energy is not provided to the atom then it won’t let the electron jump out as they are already stable. When the energy is provided the atom gets destabilized and in order to attain stability, it removes the last orbital electron(s).