Positronium may be the simplest atom that can exist, even more so than hydrogen. It is composed of two particles (particle and antiparticle, to be specific) of equal mass and opposite charge: an electron and a positron. Since there is no nucleus due to the equal masses of the positronium atom's component particles, the electron and positron orbit their center of mass.

The binding energy (energy holding the atom together) of the ground state of positronium is 6.8 electron-volts, or half that of hydrogen. Positronium occurs in two varieties, neither of which are stable:

  • Para-positronium, in which the spins of the particles are antiparallel, giving the atom a net spin of zero. Its average lifetime is approximately 10-9 seconds, at which time the electron and positron annihilate and form two gamma rays (i.e. high-energy photons).
  • Ortho-positronium, in which the component spins are aligned parallel to give a net spin of 1. After a lifetime of the order of 10-7 seconds, it decays to three gamma rays.

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