Rutherford's famous experiments in which alpha particles were scattered when they were shot into atoms led to the formulation of the nuclear model of the atom. The nucleus consists of positively charged particles called protons and uncharged particles called neutrons. The number of protons in the nucleus determines the chemical properties of the atom. The number of protons in a neutral atom is equal to the number of electrons that are in the atom. Atoms that have the same number of protons but different number of neutrons are called isotopes. Deuterium is an isotope of hydrogen that has one proton and one neutron whereas ordinary hydrogen has one proton and no neutrons. Tritium is another isotope of hydrogen that has one proton and two neutrons. The total number of protons and neutrons is the mass number of a given isotope. The notation used to identify isotopes involves writing the atomic number (number of protons) as a subscript to the left of the chemical symbol and the atomic mass (nucleon number) as a superscript to the right of the chemical symbol. Thus ₁₃Al²⁷ is an isotope of aluminum that has 13 protons and 27 nucleons, which means it has 27 - 13 = 14 neutrons.

Some nuclei are unstable and decay by emitting particles in a process called radioactive decay. Some of the particles that may be emitted were studied in the previous chapter: alpha, beta, and gamma rays. In that chapter we considered only negatively charged beta particles, which are electrons. Some nuclei emit particles that are a positively charged version of the electron called the positron.

The radioactive decay process is described in terms of how long it takes for one half of the nuclei originally present to decay - the half-life. The structure of the nucleus changes after radioactive decay. The number of nucleons (neutrons and protons) present before the decay must be equal to the number present after the reaction. We may check for this balance by determining whether the charge and mass numbers are the same before and after the decay. Thus if an alpha particle is emitted, the daughter product after the decay must have a charge that is two less than that of the original nucleus and a mass that is four less than the original nucleus. This is true because an alpha particle has a charge of two and a mass of four.

Nuclear fission is the splitting of a nucleus into parts as a result of a collision with a particle. Generally a neutron is propelled with great kinetic energy just for that purpose. Experiments show that the sum of the energies that are associated with the fission fragments is less than the energy of the original nucleus, so we conclude that the fission process results in the release of energy. This is a consequence of Einstein's theory of relativity, which will be considered in Chapter 20. The energy that is released can be used to heat water to operate a steam turbine to then generate electricity. This energy can also be used to make weapons.

Atoms consist of a very dense central core that is surrounded by electrons. There are two components that make up the central core: positively charged protons and neutrons that have no charge. The components of the nucleus are held together by nuclear forces that must be very strong to overcome the repulsion the protons experience because of their positive charge. Protons have a mass approximately 1835 times larger than that of the electron. Neutrons have a mass nearly equal to that of the proton. Neutrons have no charge, so they make convenient probes for investigating the nuclear forces. If protons were used as probes, they would be repelled by the positive charge of the nucleus and would need much higher energies to be able to penetrate the nucleus.

Radioactive decay is often described as a random process, because there is no way of predicting when a particular atom will decay. It is possible to describe the overall behavior of a collection of atoms that are decaying in terms of the number that remain after a specified period of time. This is usually expressed in terms of the half-life or the time required for one half of the original atoms to decay. The shorter the half-life the greater is the rate of radioactivity, and the longer the half-life the slower is the rate of decay.

Nuclear reactions are produced when nuclei are bombarded by other particles such as neutrons, protons, or alpha particles. We can make predictions regarding the daughter products of such reactions by ensuring that the charge number (atomic number) and mass number (nucleon number) of all the constituents before the reaction are equal to those respective quantities after the reaction. If neutrons are absorbed by an isotope of uranium, the result is the production of two fission fragments that are smaller than the uranium nucleus, the release of energy, and the production of more neutrons. If conditions are correct these neutrons may initiate additional reactions in a chain and the reaction may become self-sustaining. The fact that energy is released makes it possible to utilize the reaction to do useful work such as heating water that is used to run turbines to generate electricity or to produce nuclear weapons.

Nuclear fusion is the combining of small nuclei to form larger nuclei. If the right nuclei are combined, the energy of the product is less than the energy of the constituent parts, and energy is released just as was the case for nuclear fission. This energy has been used to produce weapons, and researchers continue to attempt to develop techniques for using nuclear fusion as an energy source. To date no economical system has been developed to achieve this, in part due to the difficulty in confining the nuclear fuel in a very small area at a very high temperature. Recent claims that nuclear fusion had been achieved at room temperatures have proved to be fraudulent.