Physically indistinguishable from the nucleus of a helium atom - two protons and two neutrons bound together by nuclear forces - but usually restricted to the product of nuclear reactions. For example, alpha particles with energies of several million electron volts are emitted in the radioactive disintegration of naturally occurring isotopes of uranium and thorium. The alpha particle carries a positive charge twice the magnitude of the electron charge. Alpha particles are important in atmospheric electricity as one of the agents responsible for atmospheric ionization. Compare beta particle, gamma ray.