Hafnium, the 72nd element in the periodic table, is chemically and physically very similar to Zirconium, the element in the table just above it. However, Hafnium's much more expensive, so Zirconium is really the go-to element in many cases. They differ in one important aspect, which leads both elements to opposite uses in nuclear reactors. Hafnium's alloys also accompanied our astronauts all the way to the Moon.
