Q.Sodium is stored in ________.

Alkali metals are chemically very reactive, and reactivity increases from Li to Cs as ionisation energy falls down the group; they react especially readily with electronegative elements such as oxygen and the halogens.

With oxygen: Li forms only the normal oxide (4Li + O₂ → 2Li₂O); Na forms mainly the peroxide (2Na + O₂ → Na₂O₂); K, Rb, Cs go further and form the superoxide (M + O₂ → MO₂). All these oxides are basic.

With hydrogen (~673 K, Li needs 1073 K): all form ionic hydrides, 2M + H₂ → 2M⁺H⁻. Reactivity toward H₂ decreases Li→Cs even as the hydride's ionic character increases and its stability falls; the hydrides are strong, increasingly powerful reducing agents down the group.

With halogens: 2M + X₂ → 2MX, all ionic except LiI (which shows covalent character, since the small, highly polarising Li⁺ distorts the large, easily-polarised I⁻); reactivity increases down the group as ionisation enthalpy falls. …