Not a fact anymore

All atoms of the same chemical element have identical mass.

What we know now

An element can contain several isotopes. Its isotopes have the same number of protons but different numbers of neutrons, giving them different masses.

Why it changed

Research on radioactivity and early mass spectrographs revealed atoms with the same chemical identity but different masses. Francis Aston then measured isotopes in many nonradioactive elements.

Status: Overturned
Category: Chemistry
Accepted for: ≈98 years
Accepted approximately: Early 19th–early 20th century
Changed approximately: 1910s–1920s

Isotopes are versions of the same element that contain the same number of protons but different numbers of neutrons, giving them different masses.

Dalton’s rule remains a useful approximation for ordinary chemistry because isotopes of an element usually behave very similarly.

Today, an element is defined by its number of protons, not by one fixed atomic mass. Periodic-table masses are often weighted averages of naturally occurring isotopes.

Sources and what they establish

Previous belief

England: John Dalton and the Atomic TheoryAmerican Chemical Society
Describes Dalton's postulate that all atoms of a given element are identical.

Historical context

Chemistry's Decision Point: IsotopesAmerican Chemical Society
Explains that equal atomic mass within an element had remained an unchallenged assumption since Dalton.
Francis Aston and the mass spectrographRoyal Society of Chemistry
Describes Aston's mass measurements and the discovery of isotopes in nonradioactive elements.

Current evidence

On This Day – Feb 18: Introduction of isotopeRoyal Society of Chemistry
Defines isotopes as atoms with the same atomic number but different mass numbers.