## The Law of Multiple Proportions

#### Learning Objective

• Define the law of multiple proportions.

#### Key Points

• The law of multiple proportions is a rule of stoichiometry.
• John Dalton formulated the law of multiple proportions as part of his theory that atoms formed the basic indivisible building block of matter.
• The law of multiple proportions says that when elements form compounds, the proportions of the elements in those chemical compounds can be expressed in small whole number ratios.
• The law of multiple proportions is an extension of the law of definite composition, which states that compounds will consist of defined ratios of elements.

#### Terms

• law of multiple proportionsA law stating that if two elements form a compound, then the ratio of the mass of the second element and the mass of the first element will be small whole number ratios.
• atomThe smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons.

## 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.