Zirconium is nearly transparent to neutrons, which makes it the perfect metal to clad nuclear fuel. But it has an evil twin, and the two are the closest pair of elements in the periodic table. In every ore on Earth, zirconium comes locked together with hafnium. Hafnium absorbs neutrons six hundred times more than zirconium does, which is enough to ruin the economics of a nuclear reactor. Hafnium is so identical to zirconium that for decades no process could separate them. Hafnium sits directly below zirconium on the periodic table, so it should be a bigger atom, but on the way there you pass through the 14 lanthanides, each adding a proton to the nucleus and an electron to a buried 4f orbital that barely shields the outer shell. The extra nuclear pull shrinks the atom by almost exactly the amount the new shell added. This identical size and charge makes these two elements very similar chemically, which is why separating them is so hard. But in the late 1940s, government chemists worked out liquid-liquid extraction schemes that could finally pry the two apart at industrial scale. Zirconium alloys as a result have become a nuclear standard, and today every commercial water-cooled reactor on Earth wraps its fuel in zirconium that has been purified in this way. And the leftover hafnium still has a use in some reactors... in control rods, to intentionally stop the chain reaction!