Here's why:

* Metallic oxides are compounds formed by the reaction of a metal with oxygen.

* Decomposition is a chemical reaction where a compound breaks down into simpler substances.

* When a metallic oxide decomposes, the oxygen atoms bonded to the metal atoms are released as diatomic oxygen molecules (O₂).

Example:

The decomposition of mercury(II) oxide (HgO) produces mercury (Hg) and oxygen gas (O₂):

2 HgO(s) → 2 Hg(l) + O₂(g)

This reaction is often used as a way to produce oxygen in the laboratory.