Scattering experiments give a strikingly simple rule: the radius grows as the cube root of the mass number, R = r₀A1/3 with r₀ ≈ 1.2 fm. Because volume ∝ R³ ∝ A, every nucleon takes up the same room — so nuclear density is essentially constant, about 2.3×10¹⁷ kg/m³, whether you weigh helium or uranium. Slide the mass number and watch the ball swell while the density needle never moves.
Add up the masses of the loose protons and neutrons, then weigh the assembled nucleus: it comes out lighter. That missing mass — the mass defect Δm = Zm_p + Nm_n − M — is exactly the energy that had to leave for the nucleus to bind: B = Δm·c² = Δm(u)×931.5 MeV. Pick a nuclide and watch the ledger; divide by A and you land back on the §03 curve.
Assembled nucleons weigh less than their loose parts — the missing mass Δm reappears as binding energy B = Δm·c². Divide by A and you get the single most important curve in nuclear physics. It climbs steeply, peaks near ⁵⁶Fe at about 8.8 MeV per nucleon, then eases down. Everything below the peak on the left releases energy by fusing; everything on the right releases energy by splitting. Slide the mass number and read which way energy wants to flow.