Alpha particles are a type of ionizing radiation discovered by Ernest Rutherford in 1898. Rutherford named these particles after alpha, the first letter of the Greek alphabet. When an atom experiences an alpha decay, it gives off an alpha particle made up of two protons and two neutrons which come directly from its nucleus. The alpha particle is the equivalent to the nucleus of the helium atom and has a mass number of 4, two protons and two neutrons. Because alpha particles have two neutrons and two protons, alpha particles have a positive electric charge.
An example of alpha decay would be seaborgium-263 which is an unstable radioactive isotope. An atom of seaborgium-263 will at some point, go through an alpha decay and give off a particle and transmute or change into rutherfordium-259 in an attempt to become stable.
Note the atomic number of rutherfordium is 2 less than seaborgium-263 because the alpha particle which has just been given off has an atomic number of 2. The mass number is 4 less because the mass of the ejected alpha particle is 4.
Radioactive decays can be written as equations similar to those used in chemistry for expressing chemical reactions.
As a general rule of thumb, you can find the progeny isotope which has just been created by an alpha decay by finding the element or isotope which has a mass number which is 4 less than the parent radioactive isotope and has an atomic number that is 2 less than the parent radioactive isotope.
Here's another example. Look on the periodic table and find astatine (mass number is 210). A radioactive isotope of astatine is astatine-211. It will decay and give off an alpha particle. Now look back two spaces on the periodic table. You will find that the element bismuth. Thus At turns into Bi. Subtract 4 from the mass number of astatine-211 and the progeny isotope is bismuth-207.
Again the equation:
Elements and isotopes that give off alpha particles are called alpha emitters. As far as subatomic particles go, the alpha particle is quite heavy so it will only travel a few centimetres in air and will not penetrate skin or clothing, posing little health risk if it remains outside the body. If alpha emitting elements enter the body through cuts or by inhaling them, however, the health risks can be quite severe. People who smoke actually inhale the radioactive isotope polonium-210, a natural occurring alpha emitter found in tobacco, into their lungs, greatly increasing the risk of lung cancer. Click here to see a list of some alpha emitters.