Dalton, in 1804, in his work on ‘Atomic Theory,’ proposed the law of chemical combination to explain how atoms form compounds. According to him, atoms of different elements combine in a simple whole-number ratio to give compounds.For example, two Hydrogen and one Oxygen combine to form water (H2O), or four Hydrogens and one Carbon combine to form methane (CH4), or one Nitrogen combines with three Hydrogens to give ammonia (NH3).