Nucleonix

Meet Nucleonix, a pair of nuclei who show how a nucleus grows as you add protons and neutrons.

Protons and neutrons are called nucleons, and the number of them in a nucleus is the mass number A. Scattering experiments show that nuclei behave roughly like tightly packed balls, with radius R = R₀A^(1/3), where R₀ is about 1.2 femtometres, or 1.2 × 10⁻¹⁵ m.

Aluminium-27 has A = 27, and the cube root of 27 is 3, so its radius is about 1.2 × 3 = 3.6 fm. A nucleus with A = 216 has a cube root of 6 and a radius of about 7.2 fm. Eight times the nucleons gives only twice the radius.

This means every nucleus has about the same density. Volume grows with R³, which grows in step with A, so adding nucleons is like adding marbles to a pile.

That density is enormous, around 2 × 10¹⁷ kg/m³, far beyond anything in everyday matter.