Think of a diatomic molecule like a pair of dancers holding hands tightly in a duo; two identical partners bonded together to move as a single stable unit.

Remember 'BrINClHOF' (Bromine, Iodine, Nitrogen, Chlorine, Hydrogen, Oxygen, Fluorine) — the classic mnemonic for all common diatomic elemental gases.

Oxygen atoms have 6 valence electrons and require 2 more electrons to complete their octet. By sharing two pairs of electrons between two oxygen atoms, they form a stable $O_2$ molecule with a covalent double bond, existing as a diatomic gas under standard temperature and pressure.

Always remember that atomicity represents the count of atoms in a single molecule. Noble gases are monoatomic, halogens and atmospheric gases like $O_2$, $N_2$, $Cl_2$ are diatomic.