Corresponding to most kinds of particles, there's an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron's the positively charged antielectron, or positron, which's produced naturally in certain types of radioactive decay. The laws of nature are very nearly symmetrical with respect to particles and antiparticles. For example, an antiproton and a positron can form an antihydrogen atom, which has almost exactly the same propertie… (More on Antiparticles)