  Berzelius himself took this latter option. He developed an atomic theory that attributed the combination of atoms in compounds to electrostatic attractions. He developed a ‘dualist’ theory to bring order to compounds involving several types of molecules. For instance, he represented copper sulphate as (CuO + SO3). Here electropositive copper combines with electronegative oxygen but in a way that leaves the combination slightly electropositive, whereas electropositive sulphur combines with oxygen in a way that leaves the combination slightly electronegative. The residual charges of the ‘radicals’ as they became known could then account for their combination to form copper sulphate.