• $summary_{type}$: concept
• $word_{count}$: 170

The nucleus contains Z protons and N = A - Z neutrons (collectively nucleons). Isotopes share the same Z but differ in A (e.g., H-1, H-2, H-3). Isobars have the same A but different Z (C-14, N-14). Isotones have the same N (C-13 and N-14 both have N=7). Nuclear radius follows R = $R_0$ * A^$\frac{1}{3}$ with $R_0$ = 1.2 fm, meaning volume is proportional to A. Since mass also scales with A, nuclear density is constant: rho = 2.3 x 10^17 kg/$m^3$ regardless of nucleus size. This is extraordinary — roughly 10^14 times denser than ordinary matter. The atomic mass unit 1 u = 1.66054 x 10^-27 kg = 931.5 MeV/$c^2$ is the standard unit for nuclear masses. Proton mass = 1.00728 u, neutron mass = 1.00866 u. The neutron-proton mass difference of 1.293 MeV drives free neutron beta decay (half-life ~10.2 min). JEE frequently tests radius ratios using R proportional to A^$\frac{1}{3}$.