$$\require{cancel}$$
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.