• Tags: nucleus, protons, neutrons, size
• Difficulty: Foundation

The nucleus contains protons (charge +e, mass 1.00728 u) and neutrons (charge 0, mass 1.00866 u), collectively called nucleons. The atomic number Z = number of protons, neutron number N = A - Z, where A = mass number = total nucleons. Isotopes: same Z, different A (e.g., C-12, C-13, C-14 — all carbon). Isobars: same A, different Z (e.g., C-14 and N-14). Isotones: same N, different Z (e.g., C-13 with N=7 and N-14 with N=7). Nuclear radius follows the empirical formula R = R_0 * A^(1/3), where R_0 = 1.2 fm. This means nuclear volume V = (4/3)piR^3 is proportional to A. Since mass is also proportional to A, nuclear density rho = 3m_p/(4piR_0^3) ≈ 2.3 x 10^17 kg/m^3, independent of A. This extraordinary density (a teaspoon of nuclear matter would weigh ~5 billion tonnes) is the same for all nuclei.