The general theme in this part of the book is again, as it was in part 6, the structure of the atom. In brief, each atom comprises a central positively charged component called the nucleus and surrounding negatively charged shell-like structures containing electrons. The nucleus is very small relative to the rest of the atom (“A fly in a cathedral”) but very massive: it contains nearly all the atom's mass. I mentioned the atomic nucleus briefly in chapter 16 as part of the heritage of Bohr and his successors, who gave us the theory of the atom's electronic domain. But the further story of the nucleus, as a separate and fundamental physical entity, remains to be told.
Reset the clock from the 1920s, where we left the development of quantum mechanics in part 6, to the 1890s, which brought the first studies of the most obvious manifestation of the atomic nucleus, radioactivity. The two great pioneers in radioactivity research were Marie Curie, in Paris, and Ernest Rutherford, first in Montreal, then in Manchester, and finally in Cambridge. Marie Curie and her husband Pierre achieved the first separation of a radioactive element, radium. Rutherford identified the three “rays” emitted by radioactive elements and called them α, β, and γ. He and his junior research partner, Frederick Soddy, demonstrated that alchemy (a more polite term is transmutation) was involved: one radioactive element could transmute into another. Using α particles (they are actually doubly charged helium ions, not rays) to bombard thin metallic foils, Rutherford, Hans Geiger, and Ernest Marsden made a strong case for the existence of the nucleus in a series of experiments completed in 1913.
Rutherford's model of the nucleus gave it an extremely small, yet finite, size. That raised the further question: What are the structural components of the nucleus? One of them was evidently the proton, the smallest nucleus, that of hydrogen. Another, the neutron, was discovered in 1932 by James Chadwick, Rutherford's second in command at the Cavendish Laboratory in Cambridge. Neutrons add mass, about that of the proton, but no electrical charge, to the nucleus.
Rutherford's favorite experimental tool was the α particle. It gave