Chemical Properties of Alkaline Earth Metals
- Alkaline-earth metals are typical divalent, and generally form colourless ionic compounds.
- Alkaline-earth metals are less reactive than alkali metals. The reactivity increases on descending the group.
- Burning in Air or Oxygen
Beryllium, magnesium and calcium burn in oxygen to form the monoxide while strontium, barium and radium form peroxides
M + O2 2 MO M + O2 MO2
(be, Mg, Ca) (Sr, Ba, Ra)
The oxides are basic in nature. These are less basic than those of Group 1 metals.
Burning in air produces a mixture of oxide and nitride.
Mg + air Mgo + Mg3N2
- Reaction with Water
Beryllium does not react with water or steam. Magnesium reacts with boiling water while the other alkaline earth metals react vigorously with cold water forming hydroxides and liberating hydrogen.
M + 2H2O M(OH)2 + H2.
- Reaction with Nitrogen
The alkaline earth metals all burn in nitrogen and form ionic nitrides.
3M + N2 M3N2.
The formation of nitride ion, N3-, from N2 requires a lot of energy as N2 is a very stable molecule. This energy is compensated by the very high value of lattice energy of M3N2 due to the high charges on the ions M2+ and N3-.
In Group 1, only lithium forms nitride. Here again the lattice energy is high due to the very small size of Li+ ion.All nitrides of alkaline earth metals decompose on heating and react with water liberating ammonia
M3N2 + 6H2O 3M(OH)2 + 2NH3.