Plutonium

Plutonium is the strangest metal known. It has six allotropes between room temperature and melting, changes volume by 25 % across them, is a poor conductor, is brittle in one phase and plastic in another, and must be alloyed with gallium to stay in a workable form — all consequences of 5f electrons sitting exactly on the boundary between localised and itinerant. Seaborg's group made the first speck in 1941; by 1945 the Hanford reactors were producing kilograms. Plutonium-238, a different isotope entirely, generates 0.57 W per gram of pure alpha heat with negligible penetrating radiation, which makes it the fuel of radioisotope thermoelectric generators — the reason Voyager 1 is still transmitting from interstellar space after 48 years, and the reason Curiosity and Perseverance can work through a Martian winter.