do elements have a variable composition
Many other elements, such. as most other nonmetals (O2, N2, S8, P4, etc.) occur as molecules. 2.6 Compounds contain atoms from two or more elements, thus the smallest unit must contain at least a pair of atoms. 2.7 Mixtures have variable composition; therefore, the amounts may vary. Compounds, as pure substances, have. constant composition, thus their composition cannot vary.