Nuclear physics is the branch of physics that studies the properties, structure, and reactions of atomic nuclei. Nuclear physics began around 1900 with the discovery of radioactivity and the nucleus. Since then, the development of increasingly powerful and precise tools has enabled physicists to study nuclei in ever greater detail.

The nucleus contains over 99.9 per cent of an atom's mass. It consists of two kinds of particles, neutrons and protons, with nearly identical masses. Protons have a positive electrical charge, and neutrons have none. The number of protons in a nucleus determines what chemical element the atom belongs to, while the neutron number determines what isotope of that element it represents.

A powerful force called the strong interaction or nuclear force binds the neutrons and protons in the nucleus and packs them closely together. Thus, all nuclei have the same extremely high density. If the earth were compressed until it had the density of an atomic nucleus, its diameter would be only about 0.5 kilometre.

Nuclei with different numbers of protons and neutrons can have remarkably different properties. Some nuclei are shaped like spheres, while others are slightly elongated or slightly flattened. Some nuclei are rigid, while others are more flexible. Some are stable, while others are radioactive and spontaneously release radiation called alpha particles, beta particles, and gamma rays in order to take a more stable form. Physicists explain these properties using sophisticated theories that allow them to predict the behaviour of new kinds of nuclei.

Much of the information about atomic nuclei comes from studies of nuclear reactions. Typically, a particle accelerator is used to create tiny, high-velocity beams of protons, neutrons, or other particles. A nuclear reaction occurs when one of these particles strikes a nucleus. Physicists then use high-precision tools to analyse the radiations emitted during the reaction.

Nuclear reactions are used in both nuclear weapons and devices developed for peaceful purposes. The fission (splitting) of very heavy nuclei and the fusion (combining) of two very light nuclei both release large amounts of energy. Usually, this energy is released in a slow, controlled way. Fission energy is used to generate electricity and to power ships, and researchers are working on devices to turn fusion energy into electricity. In nuclear weapons, very different conditions force a large number of fission or fusion reactions to occur nearly simultaneously, producing an atomic bomb or hydrogen bomb explosion.

Research in nuclear physics has led to new techniques for diagnosing and treating disease, sterilizing and preserving food, and exploring for oil. Often, these techniques make use of radioactive nuclei called radioisotopes. Nuclear research is also useful in other branches of physics and in such fields as astrophysics, biochemistry, and chemistry.