Whole-number rule

The observation that atomic masses are close to whole-number multiples of the atomic mass unit, because nuclei contain integer numbers of protons and neutrons.

Prout's hypothesis: William Prout's proposal that every element's atomic weight is an integer multiple of hydrogen's atomic weight. The whole-number pattern revived interest in Prout's proposed common atomic building block.
Atomic mass unit: A unit of mass defined as one-twelfth the mass of a neutral carbon-12 atom. The rule states atomic masses in multiples of this reference unit.
Isotope separation: Methods for enriching or isolating isotopes from a mixture of nuclides. Separating isotopes turns the rule's blurred elemental averages into distinct nuclear masses.
Integer rule: An empirical claim that atomic masses are exact integers when expressed relative to hydrogen's mass. The whole-number rule is approximate and uses the modern atomic mass unit, not an exact hydrogen-based scale.
William Prout: An English physician and chemist who proposed in 1815 that atomic weights were multiples of hydrogen's. His proposal anticipated the pattern later called the whole-number rule.
Atomic number: The number of protons in an atomic nucleus, which identifies the chemical element. It fixes one part of the nucleus's integer particle count.
Isotope labeling: The use of isotopes as tracers or markers in chemical, biological, and physical systems. Recognizing distinct isotopic masses enables tracing atoms through reactions and materials.
Isotopic mass: The mass of a specific nuclide, usually expressed in atomic mass units. It is the relevant quantity for the near-integer pattern, unlike a natural element's average.
Stanislao Cannizzaro: An Italian chemist whose 1858 pamphlet clarified how Avogadro's hypothesis could establish consistent atomic weights. Reliable atomic weights made the near-integer pattern visible across elements.
Mass number: The total number of protons and neutrons in a particular atomic nucleus. Its integer value gives the basic nuclear count behind the rule.

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Francis William AstonRelated: Aston’s measurements revealed a useful pattern in isotope masses.