In 1911, Rutherford made a major discovery at the University of Manchester. To explain the results of an experiment involving the scattering of alpha particles by a thin gold foil, Rutherford postulated his nuclear model of the atom. He recognized that the only way to explain why a few of the alpha particles scattered almost backward was to assume that almost all the mass of the atom was located in a tiny, positively charged central region he called the nucleus. And so, with this history-changing experiment, was born the concept of the nuclear atom. The Rutherford atom had a tiny central positive core that contained almost all the atom’s mass. The nucleus was surrounded by electrons in appropriate number to maintain a balance of electrical charge. The Danish physicist Niels Bohr (1885–1962) refined Rutherford’s model of the atom. In 1913, Bohr’s model of the hydrogen atom combined Rutherford’s nuclear atom with emerging concepts in quantum physics. Bohr’s innovative model marked the start of the modern theory of atomic structure. For two decades after the discovery of cosmic rays in 1911 by the Austrian-American physicist Victor Hess (1883–1964), scientists used cosmic rays as a way to study higher energy nuclear interactions. This work integrated Earth’s atmosphere as part of a natural laboratory for probing deeper and deeper into the secrets of the atomic nucleus. In a more traditional laboratory environment, Rutherford conducted an important transmutation experiment in 1919 during which he realized an emitted proton was simply the nucleus of a hydrogen atom. The following year, he suggested the possibility that a proton-sized neutral particle (called the neutron) might reside in the atomic nucleus. A little more than a decade later, in 1932, the British physicist Sir James Chadwick (1891–1974) discovered the neutron. Chadwick’s research allowed physicists to complete the basic model of the nuclear atom; namely, a central, positively charged nucleus containing protons and neutrons that was surrounded by a discretely organized cloud of orbiting electrons. The discovery of the neutron also set in motion a wave of neutron-related nuclear research in the 1930s by scientists, like Enrico Fermi, that would forever change the world.
Modern Nuclear Science: John Dalton and His School Atomic Dreams
