Reactivity with Air
Protective layers, Peroxides, and the Nitrogen reaction.
Compared to the Alkali Metals, the Alkaline Earth Metals are slightly less reactive. Their reactivity with air involves forming both oxides and nitrides, but the topmost members ($Be$ and $Mg$) display significant resistance to reaction at room temperature due to a kinetic barrier.
1. Kinetic Inertness of Be and Mg
Beryllium ($Be$) and Magnesium ($Mg$) are thermodynamically very reactive, but they appear almost inert to air and water at room temperature.
- The Protective Layer: When exposed to air, they immediately react to form a very thin, tough, and impermeable oxide layer ($BeO$ and $MgO$) on their surface.
- This layer acts as a shield, physically blocking further oxygen or moisture from reaching the bulk metal underneath.
- To make them react vigorously, they must be powdered (to increase surface area) and heated. Powdered Beryllium burns brilliantly on ignition in air.
2. Reaction with Oxygen: Oxides & Peroxides
When heated in excess oxygen, alkaline earth metals burn to form ionic oxides ($MO$). The type of oxide formed depends on the size of the cation.
Normal Oxides ($MO$)
Beryllium, Magnesium, Calcium, and Strontium primarily form regular monoxides ($MO$).
Peroxides ($MO_2$)
As we move down the group, the cations become very large. According to the size compatibility principle, a large cation can stabilize a large anion. Thus, Barium ($Ba$) and Radium ($Ra$) readily form peroxides when heated in excess oxygen.
3. Reaction with Nitrogen (The Major Difference)
Air is $78\%$ Nitrogen. In Group 1, only Lithium reacts with Nitrogen. However, in Group 2, ALL alkaline earth metals burn in nitrogen to form ionic nitrides ($M_3N_2$).
Why do they all form nitrides?
The nitride ion ($N^{3-}$) is highly charged. The Group 2 metal ions ($M^{2+}$) are also highly charged and relatively small. When they combine, the resulting crystal lattice has a massive Lattice Energy. This massive energy release is more than enough to break the extremely strong $N \equiv N$ triple bond.
When a strip of Magnesium is burned in air, it burns with a dazzling white light. Because air contains both $O_2$ and $N_2$, Magnesium reacts with BOTH.
Products: A mixture of Magnesium Oxide ($MgO$) and Magnesium Nitride ($Mg_3N_2$).
$2Mg + O_2 \rightarrow 2MgO$
$3Mg + N_2 \rightarrow Mg_3N_2$
4. Hydrolysis of Nitrides
An important chemical property of these nitrides is their reaction with water (hydrolysis). When alkaline earth metal nitrides react with water, they produce metal hydroxides and release Ammonia gas ($NH_3$).
This is a standard laboratory method to confirm the presence of a nitride (the evolved gas has a pungent smell and turns damp red litmus paper blue).
Knowledge Check
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