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 $L_n$ = $\frac{nh}{2*pi}$ (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: $E_{photon}$ = |$E_{n2}$ - $E_{n1}$| = 13.6$Z^2$|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.