Marie Curie and her two daughters, Eve (left) and Irene, in 1908.
Pierre Curie during a class lecture in 1906, the year of his death.
The gamma rays eventually turned out to be another light-like form of radiation, with waves even shorter than those of X rays. The alpha rays and beta rays, which carried electric charges, seemed to be streams of charged particles (“alpha particles” and “beta particles”) just as the cathode rays had turned out to be.
In 1900, indeed, Becquerel studied the beta particles and found them to be identical in mass and charge with electrons. They were electrons.
By 1906 Rutherford had worked out the nature of the alpha particles. They carried a positive electric charge that was twice as great as the electron’s negative charge. If an electron carried a charge that could be symbolized as -, then the charge of the alpha particle was ++. Furthermore, the alpha particle was much more massive than the electron. It was, indeed, as massive as a helium atom (the second lightest known atom) and four times as massive as a hydrogen atom. Nevertheless, the alpha particle can penetrate matter in a way in which atoms cannot, so that it seems much smaller in diameter than atoms are. The alpha particle, despite its mass, is another subatomic particle.
Here, then, is the meeting point of electrons and of atoms—the particles of electricity and of matter.
Ever since Dalton had first advanced the atomic theory over a century earlier, chemists had assumed that atoms were the fundamental units of matter. They had assumed atoms were as small as anything could be and that they could not possibly be broken up into anything smaller. The discovery of the electron, however, had shown that some particles, at least, might be far smaller than any atom. Then, the investigations into radioactivity had shown that atoms of uranium and thorium spontaneously broke up into smaller particles, including electrons and alpha particles.
It would seem, then, that atoms of these elements and, presumably, of all elements, were made up of still smaller particles and that among these particles were electrons. The atom had a structure and physicists became interested in discovering exactly what that structure was.