Helium Disguised as Hydrogen
Atoms don't have brains, so it's strange to think of one atom getting tricked by another. And yet that's exactly what scientists at the University of British Columbia in Vancouver accomplished. This bit of chemical deceit involved hydrogen and helium atoms, elements one and two on the periodic table. You might recall from high school science that the nucleus of a helium atom contains two protons and two neutrons, around which orbits two electrons. A hydrogen atom has just one proton and one electron.
Now for the trickery. Using a particle accelerator, the scientists managed to replace one of a helium atom's orbiting electrons with a muon, a subatomic particle similar to an electron, but with a mass around 200 times greater. Because it's so massive, the muon crowds the nucleus and cancels out one of the positive charges associated with the protons. What's left is a single electron zipping around a nucleus behaving as if it possessed a lone proton. In other words, the scientists successfully disguised a helium atom to look like a hydrogen atom.
Why go through this exercise? To test the effects of mass on chemical reaction rates and to confirm predictions of quantum theory. After they made the extra-heavy "hydrogen" atoms, they allowed them to bond with normal hydrogen atoms to form molecular hydrogen, or H2. They discovered that light hydrogen reacts much more quickly than the extra-heavy hydrogen made from helium, matching calculations made using quantum physics.