It quickly turned out that the atomic number was more fundamental and more characteristic of a particular element than was the atomic weight.
Niels Bohr
Bohr’s study.
Since Dalton’s time it had been assumed that all the atoms of a particular element were of equal atomic weight and that atoms of two different elements were always of different atomic weight. The first inkling and the first proof that this might not be so came through the study of radioactivity.
Isotopes
In 1902 Rutherford and his co-worker Frederick Soddy (1877-1956) showed that when uranium atoms gave off alpha particles, a new kind of atom was formed that was not uranium at all. It was this new atom that was eventually found to give off a beta particle, and then another atom of still another element was formed. This work of Rutherford and Soddy began a line of investigation that by 1907 had shown that there was a whole radioactive chain of elements, each one breaking down to the next in line by giving off either an alpha particle or a beta particle, until finally a lead atom was formed that was not radioactive.
