Law of definite proportions

The law of definite proportions states that the elements in a given chemical compound always occur in the same mass ratio. The mass ratio in sodium chloride, for example, is 39% to 61% sodium chloride.

This by Joseph -Louis Proust in 1797 established and named after him Proustsches law rule is the starting point for the development of the stoichiometry. John Dalton extended it to the law of multiple proportions.

These Proust and other observations made were an essential step in the development of the atomic theory by Dalton. Because always the same mass ratio of the elements in sodium chloride can be with her the easiest to explain: Sodium chloride is made up of equal numbers of sodium and chlorine particles. Here, a chlorine particles is about half the weight of a sodium particles.