3.2: Interpretation

We conclude that the nucleus of mass \(m\approx A m_N\) contains \(Z\) positively charged nucleons (protons) and \(N=A-Z\) neutral nucleons (neutrons). These particles are bound together by the “nuclear force”, which changes the mass below that of free particles. We shall typically write \(^{A}\)El for an element of chemical type El, which determines \(Z\), containing \(A\) nucleons.