Question

Reference Table 18.5, and give examples of the three types of alkali metal oxides that form. How do they differ?

Short answer

Three alkali metal oxides are - ${\mathrm{M}}_2\mathrm{O}$,${\mathrm{M}}_2{\mathrm{O}}_2$ and ${\mathrm{MO}}_2$. Each oxide differs in the oxidation state of oxygen, and they differ in their chemical reactivity.

Step-by-step solution

Concept Introduction

All alkali metals are soft, lustrous, and reactive but malleable. Metals react with water and oxygen and tarnish fast, thus pure metals are kept in an inert environment or beneath oil to avoid oxidation.

Examples of alkali metals oxides

Group 1A elements can form three types of oxides, ${\mathrm{M}}_2\mathrm{O}$,${\mathrm{M}}_2{\mathrm{O}}_2$and ${\mathrm{MO}}_2$.

Alkali metals are the most reactive metals on the periodic table. The valence shell electron configuration of alkali metals is ${\mathrm{ns}}^1$, thus they form ${\mathrm{M}}^{+}$ions. In each type of oxide, the oxidation state of alkali metal is the same (+1), whereas the oxidation state of oxygen differs in each type of oxide.

The oxidation state of oxygen in${\mathrm{M}}_2\mathrm{O}$is -2.

The oxidation state of oxygen in${\mathrm{M}}_2{\mathrm{O}}_2$is -1.

The oxidation state of oxygen in ${\mathrm{MO}}_2$is $-\frac{1}{2}$.

Down in the group, the ionization energy decreases. Therefore, the tendency to form the superoxide increases in the group. As less energy is available to break the double bond of the ${\mathrm{O}}_2$molecule, therefore, ${\mathrm{MO}}_2$is mainly formed with $\mathrm{K},\mathrm{Rb},$and $\mathrm{Cs}$elements.

Therefore, each oxide has a different oxidation state of oxygen and different chemical reactivity.