Definition of "alpha particle" []

• A positively charged particle, indistinguishable from a helium atom nucleus and consisting of two protons and two neutrons. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "For example, when an atom of the metal radium expels an alpha particle two protons and two neutrons, it undergoes a metamorphosis, changing from a metal to a gas known as radon."