Atomic nucleus

The atomic nucleus is the tiny, dense core of every Atom — thousands of times smaller across than the atom around it, yet holding more than 99.9% of its mass. It is built from Protons and Neutrons, together called nucleons, held against the protons' electrical repulsion by the strong nuclear force.

The shape of the idea

• The number of protons (Z) decides which element the atom is.
• The number of neutrons (N) decides which Isotope of that element it is.
• The mass number A = Z + N is the count of nucleons.

Properties

property	value
typical radius	1–10 femtometres (10⁻¹⁵ m)
share of the atom's mass	> 99.9%
size next to the atom	for hydrogen, the atom's size scale is ~63 000× the proton's charge radius
share of the atom's volume	roughly one part in 10¹⁴
held together by	the strong force, at nucleon range

The size row is CODATA 2022 arithmetic: a Bohr radius of 5.291 772 × 10⁻¹¹ m against a proton charge radius of 8.4075 × 10⁻¹⁶ m. Cube that ratio and you get the volume row. An atom is overwhelmingly empty.

Binding energy

The Nuclear binding energy per nucleon peaks near iron. That single curve explains both nuclear power sources: fusing nuclei lighter than iron releases energy (stars), and splitting nuclei heavier than iron releases energy (reactors).

Discovery

Ernest Rutherford announced the nucleus in 1911. Geiger and Marsden had fired alpha particles at gold foil; a few bounced nearly straight back, which a diffuse atom could never do.

What Rutherford actually wrote, in the Philosophical Magazine that May, is more careful than the textbook version:

Considering the evidence as a whole, it seems simplest to suppose that the atom contains a central charge distributed through a very small volume, and that the large single deflexions are due to the central charge as a whole, and not to its constituents.

Read the words again. He calls it a central charge — a phrase he uses about thirty times in that paper. The word nucleus does not appear in it once. He was claiming something narrower than "atoms have a core": he was arguing that the deflections come from the charge acting as a single object rather than from its pieces. The name, and the picture everyone now carries, came afterwards.