Atomic nucleus

The nucleus is composed of protons (positively charged) and neutrons (neutral), bound together by the strong nuclear force. The number of protons (atomic number, Z) defines the element, while the total number of nucleons (mass number, A) determines the isotope. The strong force, mediated by gluons, overcomes electrostatic repulsion between protons at short ranges. Nucleons are themselves composite particles made of quarks (up and down). The nucleus can exhibit various shapes, including spherical, deformed, and even exotic halos, as in lithium-11, where neutrons extend far beyond the core.

Several models describe nuclear behavior. The liquid-drop model treats the nucleus as an incompressible fluid, explaining binding energies and fission. The shell model, analogous to electron shells, accounts for magic numbers (2, 8, 20, 28, 50, 82, 126) that confer extra stability. The nuclear force is a residual effect of the strong interaction, with a short range (~1 fm) and a repulsive core. Modern theories, such as chiral effective field theory, derive nuclear forces from quantum chromodynamics, enabling ab initio calculations of light nuclei.

Nuclear stability depends on the balance between protons and neutrons. For light elements, the neutron-to-proton ratio is near 1; for heavier elements, more neutrons are needed to offset proton repulsion. Unstable nuclei undergo radioactive decay (alpha, beta, gamma) to reach stability. The valley of stability on a chart of nuclides marks stable isotopes. Beyond bismuth (Z=83), all nuclei are radioactive. The nuclear binding energy curve peaks at iron-56, indicating the most tightly bound nucleus; fusion of lighter nuclei and fission of heavier ones release energy.

Beyond the familiar, nuclei exhibit surprising phenomena. Halo nuclei, like helium-6, have a core surrounded by a diffuse neutron cloud, leading to a radius larger than expected. Bubble nuclei, such as silicon-34, may have a reduced central density. The nuclear skin, a thin surface region where neutron density exceeds proton density, is studied in neutron stars. The island of stability, predicted near Z=114 and N=184, suggests superheavy elements with enhanced half-lives. Nuclear isomers, metastable states like tantalum-180m, can store energy for long periods. The nuclear radius follows R = R₀A^(1/3), with R₀ ≈ 1.2 fm, but precise measurements reveal deviations due to shell effects.

Nucleon

A proton or neutron.

Strong nuclear force

The fundamental force that binds nucleons.

Isotope

Variant of an element with same Z, different A.

Magic number

Numbers of nucleons that fill nuclear shells, giving extra stability.

Radioactive decay

Spontaneous transformation of an unstable nucleus.