 A more serious challenge arose when the English radiochemist Frederick Soddy (1877–1956) established in 1913 that according to the atomic weight criterion of sameness, positions in the periodic table were occupied by several elements. Adopting Margaret Todd’s (1859–1918) suggestion, Soddy called these elements ‘isotopes,’ meaning “same place.” At the same time, Bohr’s conception of the atom as comprising a positively charged nucleus around which much lighter electrons circulated was gaining acceptance. After some discussion about criteria (van der Vet 1979), delegates to the 1923 IUPAC meeting saved the Periodic Table by decreeing that positions should be correlated with atomic number (number of protons in the nucleus) rather than atomic weight.