In the 1920s, theoretical physics seemed to flourish and move forward, while experimental physics was stalled and making no headway. Niels Bohrs creation of quantum mechanics led to new and deeper insights into forces and structures that were too small to be detected using the available experimental equipment. Excellent theoretical work could go only so far without experimental results to back it up, and there was ongoing work in England, France, and Germany to analyze the construction of the atomic nucleus as the new theories grew in acceptance and importance.

On June 3, 1920, Sir Ernest Rutherford gave the Bakerian Lecture at the Royal Society of London, on the successful transmutation of the nitro­gen atom using alpha particles. Out of character, Rutherford diverted off the topic of his nitrogen experiment into speculations concerning the constitution of the nucleus. It was known by this time that atoms were composed of a fixed number of electrons clouding a nucleus that is com­posed of a like number of protons. The negative charges of the electrons perfectly cancel the equal positive charges of the nucleus, leaving an elec­trically neutral atom. The majority of the atomic weight is due to the pro­tons, which are heavy particles, jammed closely together, making a dense nucleus. It was a workable model of the atom, and quantum mechan­ics would come to explain how chemical reactions work given no more detail than this. However, there was a serious problem. Hydrogen has one electron and one proton. Helium has two electrons and two protons. A

helium atom should weigh exactly twice what a hydrogen atom weighs. It does not. A helium atom weighs four times what a hydrogen atom weighs. If hydrogen has an atomic weight of one, then helium is four. Moreover, nitrogen has seven protons, but an atomic weight of 14, and the disparity grows worse as the atoms grow heavier. Barium has 56 protons but weighs 138. Uranium has 92 protons and weighs 235 or 238, depending on which isotope of uranium is weighed.

In his now famous lecture, Rutherford proposed a solution to this puz­zling aspect of nuclear structure. Nuclei above hydrogen are heavier than is explicable. There must be another particle at work in the nucleus. It is a particle with no measurable electrical charge, but it has all the weight of a proton. It is electrically neutral, and it should be called the neutron.

Such a particle would have interesting properties. Because it has no electrical charge, it would be free to go in and out of matter without being stopped by electron clouds covering atoms. It could not be contained by any solid walls, such as in a glass tube, or even by blocks of lead, and it would be free to enter the atomic cloud, penetrate cleanly to the center of the atom, and crash into the nucleus without being stopped. Having no charge, it would not leave an ionized trail as it flew through gas, liquid, or solid, and therefore it could not be detected with any known method of particle measurement. It was indeed an interesting particle for these reasons, but it was pure speculation. Nobody had ever seen even indirect evidence of a neutron beyond the observations of atomic weight.

In 1932, a research assistant of Rutherford’s would finally find the neu­tron, in a skillful interpretation of experimental results. From his work, experiments with the newly found particle led to the discovery of nuclear fission, and from there experimental physics took the lead in the system­atic development of nuclear power.