What is "positronium"

n. 1 (context physics chemistry English) An exotic atom consisting of a positron and an electron, but having no nucleus 2 (context physics English) An onium consisting of a positron (anti-electron) and an electron, as a particle–anti-particle bound pair.

This article is about the exotic atom. For the hydrogen isotope, see Protium. Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom, specifically an onium. The system is unstable: the two particles annihilate each other to predominantly produce two or three gamma-rays, depending on the relative spin states. The orbit and energy levels of the two particles are similar to that of the hydrogen atom (which is a bound state of a proton and an electron). However, because of the reduced mass, the frequencies of the spectral lines are less than half of the corresponding hydrogen lines.