How did the discovery of isotopes challenge John Dalton's claim that all atoms of an element have the same mass?

John Dalton’s atomic theory, formulated in the early 19th century, proposed that all atoms of a given element are identical in mass. This assumption was a cornerstone of his work, as it provided a basis for understanding chemical combinations in definite proportions.1

The discovery of isotopes challenged and ultimately disproved this claim. Isotopes are atoms of the same element that possess the same number of protons but different numbers of neutrons in their nuclei. Because the mass of an atom is determined by its protons and neutrons, isotopes of the same element have different atomic masses.234

Experimental evidence for this was provided in 1913 by J.J. Thomson, who, while studying neon, observed that neon atoms separated into different masses—specifically neon-20 and neon-22—rather than having a single, uniform mass. Consequently, the modern understanding of an element's atomic weight is not a fixed mass for every atom, but rather the weighted average of the masses of its naturally occurring isotopes.2