Uranium vs Plutonium

Uranium is a dense, silvery-grey metal that most people know for a single reason: it can be split. When a uranium-235 nucleus captures a neutron it breaks in two, releases a burst of energy and throws out more neutrons that can split further nuclei. That chain reaction runs every nuclear power station on Earth and was the basis of the first atomic weapons.

• Uranium-238 has a half-life of about 4.5 billion years, almost exactly the age of the Earth, so roughly half the uranium the planet started with is still here.
• Uranium glass, popular from the 1830s to the 1940s, fluoresces bright green under ultraviolet light, and collectors still hunt for it at car boot sales.

Few elements carry as much weight in the public imagination as plutonium. It was created in a Berkeley laboratory in early 1941, kept secret for years, and within four years had fuelled the Trinity test and the bomb dropped on Nagasaki. Yet the same element, in a different isotope, has kept the Voyager spacecraft alive for more than four decades and warms the electronics of rovers on Mars.

• Plutonium's delta phase contracts as it is heated, one of very few metals with negative thermal expansion.
• Plutonium is warm to the touch, and a sizeable piece of plutonium-238 can glow red from its own decay heat.