# What is reason of stability of metal (Sr and Ba) peroxides?

While studying inorganic chemistry I came across the following chemical equation.

$$\ce{M + Air\left(1/2O2 + N2\right) ->[Δ] MO ->[\text{Excess}~O2] MO2 (Stable) + M3N2}$$

$$\ce{M} = \ce{Sr, Ba}$$

My question is why peroxides are more stable than oxides here but in case of other alkali earth elements $$(\ce{M} = \ce{Be, Mg, Ca})$$ oxides are more stable.

These are the things that came into my mind:

The structure of $$\ce{SrO2}$$, $$\ce{BaO2}$$ are isomorphous to $$\ce{CaC2}$$. I don't think this is causing any extra stability.

The increased basic nature of oxides maybe de-stabilizing oxides when going down the group.

Or maybe the large size of $$\ce{Sr^2+}$$, $$\ce{Br^2+}$$ better fits with $$\ce{O2^2-}$$ in crystal lattice than $$\ce{O^2-}$$ which leads to increase in lattice energy of peroxides.

I am still a very beginner to inorganic chemistry so it should be helpful if someone explains in a bit more detail.

• I would wager the fact the compounds gets stabilized due to similar cation and anion sizes. – Nilay Ghosh Apr 29 '19 at 17:01
• @NilayGhosh It should be helpful if you can explain more about it. – Saurabh Singh Apr 30 '19 at 3:28
• Magnesium and calcium do form peroxides by other means, though not by direct combination. – Oscar Lanzi Apr 30 '19 at 9:24
• @OscarLanzi True, but still oxides are more stable than peroxides in case of Mg and Ca. The question is about why peroxides are more stable in case of St and Ba not whether Ca and Mg for peroxides or not. – Saurabh Singh Apr 30 '19 at 17:04