Question

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

Accepted Answer

The alkali metals emit electrons when radiation strikes on their surfaces. This phenomenon of emission of electrons when electromagnetic radiation strikes against them is called photoelectric effect. Therefore, alkali metals exhibit photoelectric effect.

The general electronic configuration of alkali metals is ${ns}^{1}$ . Alkali metals have the lowest ionization energy in each period. Within the group, the ionization energies of alkali metals decrease down the group.

The atoms of alkali metals are largest in their respective periods and therefore the valence electrons, are loosely held by the nucleus. By losing the valence electrons, they acquire stable noble gas configurations. This accounts for their case to lose electrons and hence they have low ionization energies. On moving down the group, the atomic size increases and the magnitude of screening effect (number of inner shells) also increases and consequently, the ionization energy decreases down the group.

Alkali metals have low ionization energies and therefore the electrons are easily ejected when exposed to light. Among alkali metals, caesium bas lowest ionization energy and hence it can show photoelectric effect to the maximum extent.

Embibe Experts Solutions for Chapter: The s-Block Elements, Exercise 1: Exercise-1

Embibe Experts Chemistry Solutions for Exercise - Embibe Experts Solutions for Chapter: The s-Block Elements, Exercise 1: Exercise-1

Questions from Embibe Experts Solutions for Chapter: The s-Block Elements, Exercise 1: Exercise-1 with Hints & Solutions

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