Atomic Nucleus
The extremely dense, positively charged central core of an atom containing virtually all of its mass, composed of protons and neutrons bound by the strong nuclear force.
Governing formula
R ≈ R₀ A^(1/3), R₀ ≈ 1.2 fm
SI unit
fm (Femtometers = 10⁻¹⁵ m)
In depth
Discovered by Ernest Rutherford in 1911 via alpha particle gold foil scattering. Nuclear radius scales with nucleon number A as R = R₀ A^(1/3), yielding a nearly uniform nuclear density of ~2.3 × 10¹⁷ kg/m³.
Examples in the real world
A Carbon-12 nucleus containing 6 protons and 6 neutrons packed tightly into a sphere of radius ~2.7 femtometers.