$word_{count}$: 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 = $\frac{nh}{2*pi}$ (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: $E_{photon}$ = 13.6$Z^2$|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.