KOINOT — Atomic nuclei

What to understand

Nuclear binding reflects the residual strong interaction between nucleons. The balance between attraction, electric repulsion and quantum structure determines which nuclei are stable.

Hydrogen-1 has a single proton.
Helium nuclei contain protons and neutrons.
Different proton numbers define different chemical elements.
Different neutron numbers produce isotopes.

In one sentence: Nuclear structure creates the distinct atomic species from which chemistry will later emerge.

Understand in more depth

An atomic nucleus is a quantum many-body system of protons and neutrons. Nuclear binding is not a simple pairwise force alone: spin, isospin, quantum statistics and many-body effects matter. The stability of a nucleus results from the balance between nuclear attraction, proton-proton electric repulsion and shell structure.

This is the second layer: more precise than the introductory page, but still without a full derivation.

Minimum formulas

Mass number: A = Z + N.

Binding energy: B = [Zm_p + Nm_n − M_nucleus]c².

Typical nuclear radius: R ≈ r₀A^(1/3), r₀ ≈ 1.2 fm.

History of the idea

Rutherford’s scattering experiments established the nuclear atom. The proton and neutron were identified in the early twentieth century, and nuclear shell, liquid-drop and quantum many-body models were developed to explain binding, spectra, decay and reactions.

What remains open

A fully controlled description of all nuclei directly from quantum chromodynamics remains extremely difficult. Nuclear forces, exotic nuclei, neutron-rich matter and the equation of state of dense nuclear matter are active research areas.

Where to go next

Atoms → how nuclei combine with electrons.

Stellar nucleosynthesis → how nuclei are made in stars.

Neutron stars → nuclear matter at extreme density.

Suggested sources

Standard nuclear physics textbooks.

Reviews on nuclear structure, nuclear forces and exotic nuclei.