Full Idea
In 1923 elements were defined as populations of atoms with the same nuclear charge (i.e. atomic number), allowing that atoms of the same element may have different masses.
Gist of Idea
Defining elements by atomic number allowed atoms of an element to have different masses
Source
Robin F. Hendry (Chemistry [2008], 'Chem')
Book Reference
'Routledge Companion to Philosophy of Science', ed/tr. Psillos,S/Curd,M [Routledge 2010], p.521
A Reaction
The point is that it allowed isotopes of the same element to come under one heading. This is fine for the heavier elements, but a bit dubious for the very light ones (where an isotope makes a bigger difference).