Dalton's Law
The law of multiple proportions, also known as Dalton's law, was proposed by the English chemist and meteorologist John Dalton in his 1804 work, A New System of Chemical Philosophy. It is a rule of stoichiometry. The law, which was based on Dalton's observations of the reactions of atmospheric gases, states that when elements form compounds, the proportions of the elements in those chemical compounds can be expressed in small whole number ratios.
For example, the reaction of the elements carbon and oxygen can yield both carbon monoxide (CO) and carbon dioxide (CO2). In CO2, the ratio of the amount of oxygen compared to the amount of carbon is a fixed ratio of 1:2, a ratio of simple whole numbers. In CO, the ratio is 1:1.
In his theory of atomic structure and composition, Dalton presented the concept that all matter was composed of different combinations of atoms, which are the indivisible building blocks of matter. Dalton's law of multiple proportions is part of the basis for modern atomic theory, along with Joseph Proust's law of definite composition (which states that compounds are formed by defined mass ratios of reacting elements) and the law of conservation of mass that was proposed by Antoine Lavoisier. These laws paved the way for our current understanding of atomic structure and composition, including concepts like molecular or chemical formulas.