Orthopositronium

Orthopositronium is a form of positronium, an atom-like system consisting of an electron and a positron. As the electron and positron are oppositely charged, the particle behaves like a Hydrogen atom; however, the positron has a much smaller mass than the proton it replaces.

As the electron and positron are anti-particles of each other, any form of positronium is very unstable and its lifespan is measured in nanoseconds.