PHYSICS OF THE ATOMIC NUCLEUS The nomenclature for nuclei is ^A_ZCh_N where A = N + Z.  Usually N is left off since we know it immediately from N = A - Z. Thus, ²⁷₁₃Al.

The strong nuclear force is attractive, and does not distinguish between protons and neutrons. But two effects continually fight against it... the electrical repulsion of protons, and the Pauli principle, which allows p and n into the same state, but not 2p or 2n!

The typical nuclear density is about 2 x 10¹⁷ kg/m³, while the density of a single proton or neutron is more like 2 times as great. Thus the nucleus is not like the cartoons of closely packed spheres, it is more like a compact swarm of bees.
The binding energy of nuclei can be defined as B = [Zm_p + Nm_n − M(A,Z)]c².   B/A varies from about 1 to 9 MeV for stable nuclei. The most tightly bound nucleus is ⁵⁶Fe.

The simplest possible nucleus! (click here)

What happens to a nucleus that is not stable? Nuclei with too many protons or too many neutrons are unstable to the weak nuclear force, which can change n to p, or p to n. This is called “beta decay.” Nuclei that have too many nucleons are unstable to barrier penetration, in which ⁴He clusters tunnel out. This is called “alpha decay.” Following either of these two decays, the resulting nucleus can be left in an excited state, and will transition to the ground state by emitting a photon with a kinetic energy of several MeV. This is called “gamma decay.”  Nuclei very far from stability can emit protons or He nuclei no matter how few nucleons they contain.
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