Chemical Properties of Alkali Metals
Alkali metals are the most reactive metals in the whole periodic table due to their ease in losing outermost electron hence getting oxidized easily. As the ease of losing electrons increases, the reactivity increases down the group.
Oxides
All alkali metals form oxides, which are basic in nature. Lithium forms only one type of oxide, lithium monoxide Li2O. Sodium peroxide Na2O2 is formed when sodium is heated with oxygen. Other metals of this group also form superoxides MO2 on reaction with oxygen.
4Na(s) + O2(g) → 2Na2O(s)
2Na(s) + O2(g) → Na2O2(s)
K(s) + O2(g) → KO2(s)
The formation of a particular oxide is determined by the size of the metal ion. Tiny lithium ion is not able to come in contact with sufficient number of peroxo ions. However, the ions of potassium, rubidium, caesium are large enough to come in close contact with peroxo ions and form stable structures as superoxides.
Reactivity Towards Water
Although lithium has the most negative E° (Electrode Potential), its reaction with water is considerably less vigorous than that of sodium which has the least negative E° among the alkali metals. The low reactivity of lithium is due to small size and high ionization enthalpy. All the metals of the group react with water explosively to form hydroxide and liberate hydrogen.
2M + 2H2O → 2M+ + 2OH– + H2
Basic character of oxides and hydroxides: The basic character of oxides and hydroxides of alkali metals increases with the increase in size of metal ion. So, lithium oxide and hydroxide are least basic whereas, caesium oxide and hydroxide are most basic in nature.
Hydrides
The alkali metals react with hydrogen at about 637 K to form hydrides (MH), where M stands for alkali metals.
2M + H2 → 2MH
Halides
Alkali metals react with halogens to form halides.
2M + X2 → 2MX (X = F, Cl, Br, I)