# Why is the binding energy per nucleon of helium-3 less than that of helium-4?

I'd guess it has to do with the structure of helium-3 allowing for greater Coulomb repulsion between the protons, but I'm unsure.

The Coulomb energy goes like $Z(Z-1)$, so it's typically a very small effect for light nuclei. The Coulomb energy would be responsible for the difference in binding energy between 3He and 3H.