The actinides occupy the second row beneath the main periodic table, running from actinium (Ac, element 89) to lawrencium (Lr, element 103). Like their lanthanide cousins above, they progressively fill f-orbitals — in this case, the 5f subshell. But unlike the lanthanides, every single actinide is radioactive. The first four — actinium, thorium, protactinium, and uranium — occur naturally, while all elements beyond uranium (the "transuranics") are synthetic, created in nuclear reactors or particle accelerators. Neptunium and plutonium were first produced in 1940–1941 at Berkeley, and the heaviest actinides like fermium and mendelevium have only ever been made in microscopic quantities.
Uranium and plutonium dominate the actinide story because of nuclear fission. When a neutron strikes uranium-235, the nucleus splits, releasing about 200 MeV of energy and more neutrons — triggering a chain reaction. This principle powers about 440 nuclear reactors worldwide, generating roughly 10% of global electricity with virtually zero carbon emissions during operation. Plutonium-239, bred from uranium-238 in reactors, is both a reactor fuel and the core material in most nuclear weapons. The destructive potential of actinides reshaped geopolitics: the atomic bombs dropped on Hiroshima (uranium) and Nagasaki (plutonium) ended World War II and ushered in the nuclear age.
Beyond energy and weapons, actinides have peaceful applications. Americium-241 is in virtually every household smoke detector — a tiny speck ionizes air, and when smoke disrupts the current, the alarm sounds. Thorium is being explored as an alternative nuclear fuel that produces less long-lived waste. Curium-244 generates heat through radioactive decay and powers the instruments on Mars rovers. Californium-252 is used as a portable neutron source for detecting gold and silver ores, testing bridge structures, and even treating certain cancers through neutron therapy.