Atoms, Molecules, and Ions
Interactive Student Tutorial

 2.2 Dalton's Atomic Theory and the Law of Multiple Proportions

John Dalton (1766–1844) proposed a new theory of matter that includes the following ideas:
  • Elements are composed of small particles called atoms.
  • Each element is characterized by the mass of its atoms. Atoms of the same element have the same mass; atoms of different elements have different masses.
  • Chemical combination of elements to make different substances occurs when atoms combine in small, whole-number ratios.
  • Chemical reactions rearrange the combinations of atoms; the atoms themselves are unchanged.

Dalton's theory about atoms is remarkable in that it came strictly from experimental evidence. Remember that Dalton did not know anything about an atom's structure. The law of multiple proportions is a result of Dalton's theory. It states,

Elements combine in small, whole-number multiples to form different substances.

For example, nitrogen and oxygen can combine either in a 7:8 mass ratio to form NO, or in a 7:16 mass ratio to form NO2. Comparison of the two mass ratios shows that NO2 contains exactly twice as much oxygen as NO.

 Multiple Proportions