For hydrogen-like species (one electron): r_n = 0.529 n^2/Z angstroms (radius increases with n^2, decreases with Z). E_n = -13.6 Z^2/n^2 eV = -2.18 x 10^-18 Z^2/n^2 J (energy is negative, becomes less negative with increasing n). v_n = 2.18 x 10^6 Z/n m/s (velocity decreases with n). Angular momentum L_n = nh/(2*pi) (quantised in units of h-bar). Key ratios: for different species at same n: E proportional to Z^2, r proportional to 1/Z, v proportional to Z. For transitions: E_photon = |E_n2 - E_n1| = 13.6Z^2|1/n1^2 - 1/n2^2| eV. Bohr model limitations: works only for one-electron species. Cannot explain: multi-electron atoms, fine structure, Zeeman effect, molecular bonding.