I had a conversation with my nuclear physics professor and she got mad bcs of me. I claimed there's no pi meson in the nucleus and i know a calculation (which is completed by Yukawa) that predicts an exchange particle mass about 100 MeV but a pion has a mass about 140 MeV. So I claimed that yukawa's theory is not completely correct. Where did i go wrong?? She talks something about shell ?? what did she mean? what's the complete story of Yukawa's theory???