Answer: element.
Justification:
1) A molecule is the union of two or more atoms.
2) When two or more atoms of different elements bond together, the molecule formed is a compound.
For example: one atoms of hydrogen and one atom of chlorine form the molecule HCl, which is a compound (hydrogen chloride).
3) When the molecules is formed by two atoms of the same element, it is not called a compound, but element. Some examples of this are the diatomic gases:
H₂: hydrogen
Cl₂: chlorine
Br₂: bromine
O₂: oxygen
S₂: sulfur
N₂: nitrogen.