Positronium
Positronium is an onium, a type of exotic atom that is made of a particle and its antiparticle. In Positronium, one positron and one electron are bonded together. The positron replaces a proton that would normally be present in a Hydrogen atom. They both stay in the same orbital.  The atom is unstable, and a few nanoseconds after it is created, the positron and the electron collide, destroying the Positronium and forming two gamma rays.