The law of multiple proportions says that if two elements form more than one compound together, then the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

Quick Scoop

Think of it this way:

  • Take elements A and B.
  • Fix the mass of A (say, 10 g of A).
  • In different compounds with B, the masses of B that combine with those same 10 g of A will relate as simple whole-number ratios like 1:2, 2:3, 3:4, etc.

For example, nitrogen and oxygen form several oxides; if you keep nitrogen fixed, the different amounts of oxygen that combine with it line up as 1, 2, 3, 4, 5 in ratio form, not messy decimals. This neat whole‑number pattern is one of the classic clues that elements are made of discrete atoms combining in simple integer ratios.

In short: same two elements, different compounds, and the “extra” element’s masses always compare as small whole numbers when you hold the other element’s mass fixed.

TL;DR:
When two elements make more than one compound, the masses of one element that combine with the same mass of the other are in simple whole‑number ratios.