Q

# Why are potassium and caesium, rather than lithium used in photoelectric cells?

10.9 Why are potassium and cesium, rather than lithium used in photoelectric cells?

It is because of the low ionisation energy of K and $Cs$. They lose electron easily. On the other hand, Lithium is smaller in size as compared to caesium and lithium, it requires high energy to lose an electron. Therefore, lithium is not used in a photoelectric cell.