How big is a nucleus, and why does it weigh less than its parts? Mass number and isotopes, the size and density of nuclear matter, mass defect and binding energy with E = mc², and the binding energy curve that explains both fission and fusion.
3 lessons, about 1 h 45 min3 simulations10 problems8 practice questionsPython in every lesson
Created by StemPy LabA nucleus is about m across and holds nearly all the mass of an atom. Its radius follows with fm, so its density is the same for every nucleus, about kg/m³. Isotopes share the proton number and differ in the neutron number.
A nucleus weighs less than its separate protons and neutrons. The missing mass is the mass defect , and by it equals the binding energy that holds the nucleus together. One atomic mass unit is worth MeV.
The binding energy per nucleon rises quickly to a peak of about MeV near iron and nickel, and then falls slowly. So fusing light nuclei and splitting heavy nuclei both release energy. The short-range, attractive nuclear force and the long-range electric repulsion explain the shape.
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