In 1803, John Dalton proposed a theory of matter in which the smallest particle was an atom. Dalton viewed the atom as solid and indivisible. For nearly a century, that model of the atom proved to be satisfactory for scientific work. Then, in 1897, Joseph J. Thomson discovered the first particle smaller than an atom, the electron. It was immediately apparent that atoms were not indivisible, but consisted of at least two parts. One part was the electron and the other, some positively charge particle or particles. The search for a positively-charged counterpart to the electron was not immediately successful, however.

Following up on Thomson's discovery, Ernest Rutherford bombarded a thin film of gold metal with alpha particles. He found that most alpha particles were deflected through rather small angles, as he had expected. Some, however, were deflected through relatively large angles. Indeed, a few were deflected at...