Give reasons for the following:
a. Alkli metals do not occur free in nature.
b. Alkali metal salts impart characteristic colour to the flame.
c. Caesium is used in photoelectric cell.
d. Alkali metals are good reducing agents in aqueous medium.

Explanation

a. Alkali metals are highly reactive because of their large size and low ionisation enthalpy values and hence they do not occur free in nature. They readily combine with oxygen, moisture and carbon dioxide present in air.
b. When alkali metal salts are heated in the Bunsen flame, they absorb energy. The valence shell electron gets excited to higher energy le4vel. When the exited electron returns to its original energy level, excess energy is emitted in the form of electromagnetic radiation which falls in the visible region, thereby imparting colour to the flame. Since the ionisation energy decreases form Li to Cs, the amount of energy released increases form Li to Cs. Thus, the frequency of light emitted increases in accordance with the formula E= hv. Hence, different alkali metals import charactercistic colour to the flame.
c. Because of low ionisation enthalpy, caesium is used in photoelectric cells, since low-energy photons (light) can eject electrons (s) form the surface.
d. Alkali metals are good reducing agents because they can lose electron(s) easily on account of low ionisation enthalpy values.