Could someone list the similarities and differences between the two? Is it correct to say that 'the set of all molecules' is a subset of 'the set of all compounds'. But the vice versa is incorrect. So a 'proper' should be placed in front of the word 'subset' above. i.e. any covalently bonded molecule is considerered a compound but an ionic compound like NaCl cannot be a molecule because 1) It dosen't have a definite arrangement 2) There is no smallest particle of an element or compound that retains the chemical properties of the element or compound. The whole 'infinite' lattice must be considered which cannot be a molecule.