Question

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

Class XI The s-Block Elements Page 305

Answer 1

Concpet:- metal have high tendency to lose electrons.(lower the ionisation energy, higher is the tendency to lose electrons) that's why metals are used in photoelectric cells.

Potessium and Caesium have much lower Ionisation enthalpy than that of Lithium . Therefore ,these metals on exposure to light emits electrons easily but lithium doesn't . That's why K and Cs rather than Li are used in photoelectric cells.

Answer 2

Answer:

This is true. Photoelectric cells are made up of elements which readily give out electrons when sunlight falls on them.

Potassium and Caesium are larger in size compared to Lithium. Moreover, Potassium and Caesium have shielding effect due to which electrons in the outermost shell can escape easily which yields for the low ionisation power of them.

But Lithium has a small size and the electrons are attracted by a great effective nuclear charge due to which, electrons cannot escape.

Hence Lithium is not used in photoelectric cells.