The 1930s saw the birth of nuclear physics - a new science that
was to have profound consequences. The structure of the atom, with electrons
surrounding a tiny, central nucleus, became clear around 1911-13, but it was
only with an understanding of the structure of the nucleus itself that scientists
could harness the great energies locked at the heart of the atom.The discovery of the nucleus showed that most of an atom's mass and all its positive electric charge are concentrated in a small central region. And by 1919 Ernest Rutherford had found that the nuclei of several elements contain positively-charged particles of matter identical to the nucleus of hydrogen, the lightest atom. He argued that these particles are constituents of all nuclei, and he named them "protons", for the first nuclear particles. Thirteen years later the picture was completed with the British physicist James Chadwick's discovery of the neutron – an electrically neutral particle, only slightly heavier than the proton.
Together protons and neutrons form the atomic nuclei of all elements (except hydrogen, where the nuclei are single protons). Scientists now know that they are bound together by the so-called strong nuclear force. In any atom the number of electrons - which determines the chemical properties of the element – exactly balances the number of protons in the nucleus. The role of the neutrons is to dilute the repulsive electric force between the protons, and so prevent the nucleus from flying apart. In larger nuclei, an increasing number of neutrons is necessary to counteract the electric force, so that in the heaviest nuclei the number of neutrons greatly exceeds the number of protons.
Only certain configurations of protons and neutrons prove to be completely stable, however. Others give rise to nuclei that are unstable, or, in other words, radioactive. They change to more stable structures through the spontaneous emission of radiations - alpha particles (helium nuclei), beta particles (electrons) and gamma rays (high-energy photons). And in some instances a large nucleus, such as uranium, can break into two more or less equal fragments and a few neutrons in a process known as fission.
The energies released in these transmutations are millions of times greater than those involved in chemical reactions. This is due to the strength of the nuclear binding force. In the late 1930s, after the discovery of fission in uranium bombarded by neutrons, physicists realized that they could set up a chain reaction, with the neutrons released in one fission producing another and so on. When controlled, this chain reaction leads to a useful source of power; when uncontrolled it leads to devastating explosion.
Physicists soon saw the potential uses and misuses of this source of nuclear energy. Their greatest dreams have come to life with the development of reactors to generate nuclear power; their worst nightmares are realized with the proliferation of nuclear weapons (Science A History of Discovery in the Twentieth Century)