Question

Potassium and caesium used in photochemical cells why?

Answer 1

Potassium and Cesium have low ionization energies. So, a low energy photon can also emit an electron from their atoms. Thus, they are used in photoelectric cells. Sodium and lithium have larger ionization energies as they are smaller in size and have higher effective nuclear charge. So, removing an electron from them requires higher energy and hence they are not used in photoelectric cells.