word_count: 220

Bohr's model applies to one-electron species (H, He+, Li^2+). Key equations: r_n = 0.529 n^2/Z angstroms (radius), E_n = -13.6 Z^2/n^2 eV (energy), v_n = 2.18 x 10^6 Z/n m/s (velocity). Angular momentum L = nh/(2*pi) (quantised). Energy is negative (bound state), zero at n = infinity (ionisation). Ground state of H: E = -13.6 eV, r = 0.529 A, v = 2.18 x 10^6 m/s. For He+: E = -54.4 eV, r = 0.265 A. In transitions: E_photon = 13.6Z^2|1/n1^2 - 1/n2^2| eV. KE = -E (positive), PE = 2E (negative), so |PE| = 2KE. Bohr radius (a_0 = 0.529 A) is the most probable distance for 1s electron in quantum mechanics. Key ratios for comparison: E proportional to Z^2/n^2, r proportional to n^2/Z, v proportional to Z/n. Bohr model limitations: fails for multi-electron atoms, cannot explain fine structure, Zeeman effect, or molecular bonding. It was a crucial stepping stone from classical to quantum physics.