Law of multiple proportions

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

Atomic theory: The theory that matter consists of atoms, which combine in definite ways to form substances. Its discrete atoms explain why combining masses fall into whole-number ratios.
John Dalton: An English chemist and physicist who developed an influential early atomic theory. Dalton used measured combining ratios to support his account of atoms.
Carbon monoxide: A compound of carbon and oxygen with the formula CO. Compared with carbon dioxide, it demonstrates a one-to-two oxygen mass ratio for fixed carbon.
Law of combining volumes: The law that reacting gas volumes, measured under the same conditions, combine in simple whole-number ratios. It describes ratios of gas volumes, not masses in compounds.
Chemical formula: A notation showing the elements in a substance and their relative numbers of atoms. Formulas reveal the atom-count ratios behind a compound’s mass composition.
A New System of Chemical Philosophy: John Dalton’s book, published in parts from 1808, presenting his atomic theory and chemical data. It set out Dalton’s explanation of multiple proportions through combinations of atoms.
Carbon dioxide: A compound of carbon and oxygen with the formula CO₂. Its composition can be compared with carbon monoxide to illustrate the law.
Avogadro’s law: The principle that equal gas volumes at the same temperature and pressure contain equal numbers of molecules. It explains simple gas-volume ratios through particle counts rather than comparing compound masses.
Atomic mass: A measure of an atom’s mass, commonly expressed relative to one-twelfth the mass of carbon-12. Atomic masses convert whole-number atom counts into the measured mass ratios.
Joseph Proust: A French chemist known for establishing the law of definite proportions through chemical analyses. Proust’s work on fixed composition helped establish the experimental context for Dalton’s law.

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