The atomic nucleus is consists of a combination of protons, which have a positive electrical charge, and neutrons, which are electrically neutral. Protons and neutrons are both called nucleons. Every atomic nucleus contains at least one proton, meaning that the nucleus itself has a positive charge. Except for hydrogen, all naturally occurring atomic nuclei also contain at least as many neutrons as protons. For elements larger than calcium, which has 20 protons, all naturally occurring nuclei have more neutrons than protons. The excess of neutrons increases with the size of the nucleus. Nuclei with fewer neutrons than these natural limits have been made artificially, but they are all very unstable and decay rapidly. Very large nuclei are also unstable. Unstable nuclei can emit energy in the form of gamma rays or particles such as protons, neutrons, alpha particles (two protons and two neutrons), and other nuclei.
In nuclei containing more than one proton—that is, every element other than hydrogen-each proton repels every other proton by the electromagnetic force. However, a very powerful force, known as the nuclear, or strong, force also affects the nucleons. This force acts at an extremely short range, about one femtometer (10-15 m). The strong force is complicated and only partially understood. At very small distances between nucleons, less than one femtometer, the force is repulsive, pushing the nucleons apart. At distances of about one to two femtometers, roughly the diameter of a nucleon, the force is strongly attractive and binds nucleons together by energy millions of times stronger than the electromagnetic repulsion of charged protons. Beyond a few femtometers, the attraction of the strong force drops off rapidly with distance, having a much smaller effect than the nuclear particles than does the electromagnetic force. Because the particles of the nucleus are bound so closely together, the volume of the nucleus is very small compared to that of the atom itself. Although most of the atom's mass in the nucleus, the nuclear volume is only about one ten-thousandth of the atom.
Since the strong force acts at a short range, each nucleon is attracted only by its immediate neighbors. The amount of attractive energy within the nucleus increases in proportion to the number of nucleons. Because each proton interacts with every other proton in the nucleus, whether they are adjacent or separated by other nucleons, the repulsive electromagnetic energy increases as a square of the number of protons in the nucleus. In large nuclei, this electrical repulsion becomes strong enough to make the nucleus unstable. Fission occurs when the nucleus divides into two smaller nuclei that are then rapidly driven apart by electrical repulsion. This division can occur spontaneously or as the result of added energy of a collision of a rapidly moving particle, such as a neutron or another nucleus. A chain reaction occurs when the fission process releases energetic particles that strike other nuclei, releasing energy and more particles. All nuclei containing more than 240 nucleons are unstable, as are many smaller nuclei. The energy released during a nuclear reaction is about one million times as strong as that of a chemical reaction, due to the relative size of the nucleus to the atom and the magnitude of the strong force at nuclear distances.
The effects on the nucleons of the strong nuclear is also the source of energy release by nuclear fusion, the process that fuels the sun. Two nuclei of deuterium, hydrogen molecules with one proton and one neutron each, combine to form a helium nucleus. A very large amount of energy is released as the nuclear force pulls the pairs of nucleons together. Initially, energy is required to overcome the electrical repulsion of the protons. In the sun, this energy is supplied as kinetic energy when the nuclei are heated to temperatures of millions of degrees.
Nuclear accelerators are important tools for the study of the nucleus. They create beams of high-energy particles such as electrons, nucleons, alpha particles, or larger nuclei. The particles are accelerated to velocities close to the speed of light. At this velocity, the particles have sufficient energy to overcome the effects of electromagnetic forces and initiate nuclear reactions when they strike target nuclei. The products of these reactions, particles and energy, disclose information about the nature of the nucleus and the forces affecting it. In some cases, the accelerated particle combines with the target nucleus, creating a larger nucleus. This is the source of elements and isotopes that do not occur in nature, such as the transuranium elements.
The strong force can be studied by observing nucleon interactions when a nucleus is bombarded with a beam of protons or neutrons. The effect of the strong force deflects the beam particles passing within a certain distance of the nucleons of the target. By observing these deflections, physicists have calculated the diameter of the nucleons and the intensity and reach of the strong force. Other information of the strong force has been derived from the electromagnetic radiation and particles emitted by the nucleus during transitions between quantum states of the nucleus. The energy difference between the states is equal to the energy emitted as very high energy, short wavelength gamma rays.
According to the big bang theory, immediately after the big bang, the universe consisted of an unimaginably hot plasma of quarks and gluons, the components of nucleons. After approximately one one-millionth of a second, these particles began to combine in groups of three quarks, accompanied by gluons, to form neutrons and protons. In about three minutes, these nucleons began to join together to form atomic nuclei--hydrogen, helium, and a small amount of lithium. Twelve minutes into the life of the universe, it had cooled enough that nucleons no longer had sufficient energy to join together. The larger nuclei did not form until much later, when they were produced in the hot interior of stars. Experiments using large accelerators try to duplicate the density and temperature of the early universe in order to produce conditions that will allow the quarks in individual nucleons to exist separately. These results would provide information the structure of the nucleus and the nature of the strong force as well as insight into the beginning of the universe.
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