This particular subdivision of molecules contains two atoms that are chemically bonded. If the atoms are the same, or identical, they are a homonuclear diatomic molecule. The best known example of this is probably oxygen, since after all, we breath it in every day.
If the two atoms are different, they compose a heteronuclear diatomic molecule. Molecules like this form by combining atomic orbits of unequal energy. This results in a polar bond, in which atomic orbitals contribute differently to each molecular orbit. Generally, heteronuclear molecules with energy difference greater than ten eV do not combine in a favorable way.
Where Are They?
Diatomic molecules are all around us. Hydrogen is in fuel cells. Nitrogen is used to prolong the shelf life of perishable foods. Chlorine is used for water treatment. Most importantly, oxygen allows the people and animals on Earth to breath. Although we can’t see them, these molecules play pivotal roles in the world we live in.
In conclusion, diatomic molecules are an important subdivision of molecules.