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Alpha

Radiation and the atom · level 5, lesson 2 of 6

A question to start

Alpha particles cannot get through a sheet of paper, or even the dead outer layer of your skin. So why are they thought to be the most harmful kind inside the body?

The idea

An alpha particle is 2 protons and 2 neutrons stuck together: the nucleus of a helium atom. It is heavy and has a charge of +2. Alpha particles come from very heavy nuclei, like uranium, radium, radon and polonium.

Because they are big and charged, they crash into the atoms around them almost at once and give up all their energy in a very short distance: a few centimetres of air, or a sheet of paper. Outside the body, the skin stops them. But if an alpha source is breathed in or swallowed, all that energy goes into a few cells, causing much more damage than beta or gamma.

When a nucleus gives out an alpha particle, it loses 2 protons and 2 neutrons:

  • the atomic number goes down by 2;
  • the mass number goes down by 4.
\[{}^{238}_{92}\text{U} \;\to\; {}^{234}_{90}\text{Th} + {}^{4}_{2}\text{He}\]
uranium-238thorium-234alpha

Worked examples

1. Uranium-238. \(Z\): \(92 - 2 = 90\), which is thorium. \(A\): \(238 - 4 = 234\). So uranium-238 becomes thorium-234.

2. Radon. Radon-222 has 86 protons. After an alpha decay: \(Z = 84\) (polonium) and \(A = 218\): polonium-218.

3. Smoke alarms. Many smoke alarms hold a tiny speck of americium-241, an alpha source. Its alpha particles let a small current flow through the air in a little chamber. Smoke gets in the way, the current drops, and the alarm sounds.

Try it

  1. What is an alpha particle made of?
  2. Why can alpha particles not get through skin?
  3. Radium-226 has 88 protons. What does it become after an alpha decay?
  4. Why is radon gas in a home a concern, when alpha particles cannot pass through skin?
Answers
  1. 2 protons and 2 neutrons: a helium nucleus.
  2. They are heavy and charged, so they lose all their energy almost at once.
  3. \(Z = 86\), \(A = 222\): radon-222.
  4. It is breathed in, so its alpha particles hit the cells of the lungs from inside.

Common mistake

"Alpha is the weakest, so it is the safest." It is the easiest to stop, but inside the body it is the most damaging, because it dumps all its energy in a few cells.

Where it leads