Nucleus
The dense, positively charged core of an atom, containing virtually all of its mass in protons and neutrons.
Formula / equation
r ≈ 1.2 × A^(1/3) fm
Unit
m
In depth
Although it occupies about one part in 10¹⁵ of the atom's volume, the nucleus holds more than 99.9% of its mass, giving nuclear matter a density near 2 × 10¹⁷ kg/m³. The strong nuclear force binds nucleons together against electrostatic repulsion but acts only over about 10⁻¹⁵ m.
Examples in the real world
Rutherford's alpha-scattering experiment revealed the nucleus when a few particles rebounded from gold foil.