Atomic nucleus

Structure and size

Although the nucleus occupies a tiny fraction of an atom's spatial extent, it contains nearly all of the atom's mass. Nuclear sizes are measured in femtometers (1 fm = 10−15 m) and typically range from about a femtometer for the lightest nuclei up to a few dozen femtometers for the heaviest naturally occurring elements. For example textbooks often contrast the tiny core of hydrogen with the larger cores of heavy elements such as hydrogen and uranium. Compared with the overall atomic radius, the nucleus is smaller by many thousands of times, but it concentrates nearly all nuclear mass in that small volume as summarized in surveys of atomic mass. The mass contribution of orbiting electrons is comparatively negligible.

Forces and stability

Protons carry positive electric charge while neutrons are electrically neutral. Electrostatic repulsion between protons would make a multi‑proton assembly unstable if not for a much stronger interaction that acts only at short range. This short‑range attraction, commonly called the strong nuclear force, overcomes repulsion and binds nucleons together. Nuclear binding energy is the energy required to separate a nucleus into its individual nucleons; it is central to understanding why some nuclei are stable and others undergo spontaneous transformation.

Isotopes, decay and reactions

Atoms of the same element can have different numbers of neutrons; these variants are called isotopes and share chemical properties while differing in nuclear mass and stability. Unstable nuclei may decay by emitting particles or radiation (alpha, beta, gamma), a process that transforms one nuclidic species into another and changes nuclear energy. Nuclei also participate in induced processes: fission splits a heavy nucleus into lighter parts and releases energy, while fusion combines light nuclei to form heavier ones, powering stars and experimental reactors.

History and concepts

Early usage of the word "nucleus" predates modern atomic theory: Michael Faraday used the term in the 19th century to refer to central parts of atoms, and the modern nuclear concept was developed through the Rutherford scattering experiments in the early 20th century. Key historical references include Faraday and the experimental work led by Rutherford, which established that most of an atom's mass is concentrated in a tiny central region.

Importance and applications

• Fundamental to chemistry: the nucleus determines an element's identity through its proton number.
• Energy and technology: nuclear fission and fusion release large amounts of energy and are used or studied for power generation and propulsion.
• Medicine and dating: radioactive isotopes have diagnostic and therapeutic uses in medicine and are tools for dating archaeological and geological samples.
• Research: studies of nuclear structure and reactions inform particle physics, astrophysics, and materials science.

For additional background and technical summaries consult introductory resources on the nucleus, on nucleons, and on the roles of protons and electrons in atomic structure. Historical discussions of the terminology and experiments that shaped the field mention figures such as Faraday and Rutherford, while modern treatments address isotopes, binding energy and nuclear forces in greater detail.