When two elements form more than one compound, masses of one combining with a fixed mass of the other are in small whole-number ratios.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios. Dalton noted this pattern in 1804 and used it as strong evidence that matter is made of discrete atoms. The pattern follows naturally if each compound contains a specific whole number of atoms of each element. It distinguishes the different compounds formed by the same pair of elements, such as the oxides of carbon or nitrogen. The ratios are always simple fractions, never arbitrary values, which is why they reveal an underlying atomic count.
Key ideas: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios (Dalton). For CO and CO2, 12 g of C combines with 16 g and 32 g of O, a 1:2 ratio. Method: fix the mass of one element, then divide the other masses.
Common mistakes and connections: Students compare the ratio of raw masses between compounds instead of fixing one element first. It applies to different compounds of the same two elements only, not to unrelated compounds. It was key evidence that atoms combine in whole numbers, and it prepares you for oxidation states and formulas like N2O, NO, NO2.
Carbon forms CO and CO2; the oxygen combining with 12 g of carbon is 16 g and 32 g, a simple 1 to 2 whole-number ratio. The interactive lesson shows this as a live animation or chart.
Carbon forms CO and CO2: per 12 g of carbon there are 16 g and 32 g of oxygen, a simple 1 to 2 mass ratio.
When two elements form more than one compound, masses of one combining with a fixed mass of the other are in small whole-number ratios.
Law of Multiple Proportions is a intermediate topic in Atomic Structure & Periodicity. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Atomic Structure & Periodicity first: Chemical Change, Law of Conservation of Mass, Law of Definite Proportions.
The next topics in Atomic Structure & Periodicity are Dalton's Atomic Theory and Electron.
Yes. The interactive lesson includes a self-check question on Law of Multiple Proportions, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Law of Multiple Proportions is topic 11 of 115 in Atomic Structure & Periodicity.