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An atom composed of antiparticles. An antiatom consists of positrons, antiprotons, and antineutrons. It has the same mass and spin as an ordinary atom, and the same amount of charge and magnetic moment, but the charge and magnetic moment are the opposite of those of an ordinary atom.
It has been known for decades that each fundamental particle in nature has its antiparticle. The first antiparticle to be discovered, in 1932, was the positron, identical to the negatively charged electron but having a positive electric charge. The negatively charged antiproton, the antiparticle of the positively charged proton, was first produced in 1955. The existence of a whole set of antiparticles argues for the possibility of antiatoms and bulk antimatter, identical to atoms and "normal" matter except for the reversal of electric charge and certain other quantum properties of its constituent particles. On the other hand, it is a fact that the collision of a particle and its antiparticle results in the immediate annihilation of both particles. Thus, although it may be easy to envisage an atom of the simplest element, hydrogen (made up of one proton and one electron), matched by an atom of antihydrogen (made up of an antiproton and a positron), the antiatom would survive only as long as it did not meet a normal atom.