When a substance undergoes alpha radiation it gives off alpha particles. An alpha particle is made up of 2 protons and 2 neutrons. This means that an alpha particle has a mass of 4 and a charge of +2. An alpha particle can be written three ways:

We use the symbol for helium because an alpha particle is the same as a helium nucleus.

Source: Alpha particle radiation, University of Michigan Student Chapter of the Health Physics Society

The diagram above shows the original nucleus (parent) of unstable uranium-235. U235 naturally gives off two neutrons and two protons which is the same as a helium nucleus which has a positive charge. By losing the two protons uranium, which has 92 protons in the nucleus, it changes into thorium–231, which has 90 protons in the nucleus.

Other elements which are alpha emitters are:

Americium-241, used in smoke detectors

Californium-252, used to inspect luggage for explosives

Radium-226, used to make lightning rods more effective

Thorium-220, used for coloring in glassware

Uranium-235, used in nuclear power plants

Alpha particles are relatively heavy and large. So as they travel through air, the distance they can travel is just a few centimeters. Because of their size alpha particles are easily stopped by a single sheet of paper as demonstrated in the graphic below.

Source: Penetrating distances, University of Michigan Student Chapter of the Health Physics Society

Alpha particles cannot penetrate the outer layer of the skin. So they do not present a hazard from exposure from outside the body. However, if ingested or inhaled they can become very dangerous by penetrating soft tissues inside the body.

Click to Run

Source: Alpha Decay, PhET, University of Colorado at Boulder

1. Watch alpha particles escape from a polonium nucleus, causing radioactive alpha decay. See how random decay times relate to the half-life.
2. Click on "add 10" to add more Polonium to the demonstration.
3. You can "Pause" the sim and then use Step to incrementally analyze what is going on in the decay.

Now answer the following questions.

1. What element isotope do you start with (the parent)?
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   Check Your Answer

   Polonium-211
2. What element isotope do you end up with?
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   Lead-207
3. How many neutrons does the parent isotope lose?
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   2, since it loses the equivalent of a helium nucleus.
4. How many protons does the parent isotope lose?
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   Check Your Answer

   2, since it loses the equivalent of a helium nucleus.