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

Step 1: Recall the photoelectric effect requirement

A photoelectric cell works by using incident light (photons) to eject electrons from a metal surface. The lower the energy needed to remove an electron (i.e. the lower the ionisation enthalpy/work function), the more efficiently ordinary visible light can do this.

Step 2: Compare ionisation enthalpies within the group

Ionisation enthalpy decreases down Group 1 as atomic size increases and the valence electron becomes more shielded and farther from the nucleus. Potassium and caesium, being much larger atoms than lithium, have significantly lower ionisation enthalpies.

Step 3: Apply this to the photoelectric cell …

Connect ionisation enthalpy (which falls down Group 1 with increasing atomic size) to how easily visible-light photons can eject an electron — K and Cs, having the lowest IE among common ch …

• Explaining this purely in terms of reactivity with water/air rather than the relevant property (ionisation enthalpy/ease of photoelectron ejection). …