The law of multiple proportions says the masses of one element combining with a fixed mass of another form:

1. Two elements can react to form more than one compound. 2. The law of multiple proportions states that a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers. 3. Copper and chlorine form a green solid at 0.558 g chlorine per 1 g copper and a brown solid at 1.116 g per 1 g copper. 4. Taking a ratio of those ratios gives a small whole number, in this case two to one. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 2 'Atoms, Molecules, and Ions', sections 2.1-2.6_