Subatomic Particles: Protons (p⁺) determine atomic number $Z$ and element identity; Neutrons (n⁰) stabilize the nucleus via the residual strong force; Electrons (e⁻) orbit in quantized shells $n=1..7$.
Nuclear Stability: Stable light nuclei require $N/Z \approx 1.0$, increasing to $N/Z \approx 1.54$ for heavy elements. Nuclei outside the Valley of Stability decay via $\beta^-$, $\beta^+$/EC, or $\alpha$ emission.
Binding Energy (SEMF): Calculated via the Bethe-Weizsäcker Liquid Drop formula ($E = \Delta m \cdot c^2$). $B/A$ peaks at $^{56}\text{Fe}$ (~8.79 MeV/nucleon).