Nuclear physics

Radioactive decay, binding energy, fission, and fusion

What is nuclear physics?

Nuclear physics studies the nucleus at the center of the atom — the tiny, dense core made of protons and neutrons — and the forces and processes that hold it together or cause it to break apart.

Inside the nucleus

Every atom has a nucleus made of protons (positively charged) and neutrons (no charge), collectively called nucleons. The number of protons determines which chemical element the atom is; the number of neutrons can vary somewhat for the same element, producing different isotopes of it. Even though protons all repel each other electrically (like charges repel), the nucleus doesn't fly apart, because a separate, much stronger force — the strong nuclear force — binds nucleons together at very short range, overpowering that electric repulsion.

Radioactivity: unstable nuclei

Some combinations of protons and neutrons are unstable — the nucleus will eventually rearrange itself to become more stable, releasing energy and particles in the process. This is called radioactive decay. There are a few common types: alpha decay (the nucleus ejects a small cluster of two protons and two neutrons), beta decay (a neutron converts into a proton, or vice versa, emitting an electron or its antiparticle), and gamma decay (the nucleus releases pure energy as a high-energy form of light). Every radioactive isotope decays at a fixed, statistically predictable rate, described by its half-life — the time it takes for half of a sample to decay.

Nuclear fission and fusion

Fission is the splitting of a large, heavy nucleus (like uranium) into smaller pieces, releasing a large amount of energy — this is the process that powers nuclear reactors. Fusion is the opposite: combining small, light nuclei (like hydrogen) into a larger one, also releasing energy — this is the process that powers the Sun and other stars. Both processes release far more energy per reaction than ordinary chemical reactions, because they tap directly into the strong nuclear force and the mass-energy relationship E=mc² described in the Relativity article.

Why nuclear physics matters

Nuclear physics underlies nuclear power generation, medical imaging and cancer treatment using radioactive isotopes, radiocarbon dating of ancient artifacts, and our understanding of how the Sun and stars generate their energy.