Actinoids (Ac-Lr, Z=89-103) have configuration [Rn]5f^(0-14) 6d^(0-1) 7$s^2$. All are radioactive; elements after uranium (Z>92, transuranium) are synthetic. Show wider range of oxidation states (+3 to +7) than lanthanoids because 5f, 6d, and 7s energies are comparable. U shows +3 to +6 (UF6 = +6, volatile, used in uranium enrichment by gaseous diffusion). Actinoid contraction is greater than lanthanoid contraction because 5f electrons shield even more poorly than 4f. Key differences from lanthanoids: (1) more variable oxidation states, (2) form more oxocations like $UO2^{2+}$ (uranyl), (3) greater tendency to form complexes, (4) most are radioactive and have no stable isotopes.