Cl₂ is a compound
Further explanation
Pure substance can be any element or compound
Elements if composed of one atom
A compound if it is composed of two or more atoms, either of the same type or of different types
Cl₂ consists of 2 Cl atoms.
Cl with 7 valence electrons will try to get its stability by binding 1 more electron, by covalently bonding with other Cl atoms to form a Cl₂ compound in the form of a gas