Whole number rule

"[4] The law of definite proportions was formulated by Joseph Proust around 1800[5] and states that all samples of a chemical compound will have the same elemental composition by mass.

[9] In 1920, Francis W. Aston demonstrated through the use of a mass spectrometer that apparent deviations from Prout's hypothesis are predominantly due to the existence of isotopes.

A secondary cause of deviations is the binding energy or mass defect of the individual isotopes.

[11][12] In 1932, James Chadwick discovered an uncharged particle of approximately the mass as the proton, which he called the neutron.

[13] The fact that the atomic nucleus is composed of protons and neutrons was rapidly accepted and Chadwick was awarded the Nobel Prize in Physics in 1935 for his discovery.

Francis W. Aston received the 1922 Nobel Prize in Chemistry for his enunciation of the whole-number rule.
John Dalton's list of atomic weights and symbols
James Chadwick, discoverer of the neutron, with General Leslie Groves , director of the Manhattan Project .