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Radioactive decay

Radiation and the atom · level 5, lesson 1 of 6

A question to start

In 1896, Henri Becquerel left some uranium salt on a wrapped photographic plate in a drawer. When he developed the plate, it was fogged, though no light had reached it. What had?

The idea

The uranium was giving out invisible radiation of its own. Marie Curie named this radioactivity. With Pierre Curie she went on to find two new, far more radioactive elements in 1898: polonium and radium.

Radioactivity comes from unstable nuclei. Sooner or later such a nucleus breaks down, or decays, and throws out radiation. Often it turns into a different element. There are three main kinds:

Radiation What it is Stopped by
Alpha (α) 2 protons and 2 neutrons a sheet of paper
Beta (β) a fast electron a sheet of plastic or thin aluminium
Gamma (γ) a high-energy photon of light reduced by thick lead or concrete

All three are ionising: they can knock electrons out of atoms. In a living cell this can damage DNA, which is why radiation can be harmful.

Radioactive atoms are everywhere in small amounts: uranium, thorium and potassium in rocks, and radon gas that seeps up from the ground. On average, radon is the biggest natural source of radiation that people receive.

alphabetagammapaperaluminiumlead

Worked examples

1. Becquerel's plate. The plate was wrapped, so the radiation from the uranium passed through the paper wrapping. (Alpha would be stopped by the paper, so some of the uranium's radiation must have been beta or gamma.)

2. Ionising. Ultraviolet light from Level 2 can damage skin. Alpha, beta and gamma carry much more energy each, enough to knock electrons right out of atoms.

3. A changing element. Radium decays to radon. Radium has 88 protons and radon 86, so the nucleus has lost 2 protons.

Try it

  1. Who discovered radioactivity, and when?
  2. What happens to an unstable nucleus?
  3. Which kind of radiation does paper stop?
  4. Why can radiation damage living cells?
Answers
  1. Henri Becquerel, in 1896.
  2. Sooner or later it decays, throwing out radiation and often becoming a different element.
  3. Alpha.
  4. It knocks electrons out of atoms, which can damage the DNA in cells.

Common mistake

"Radioactive things glow." Most radioactive materials look completely ordinary. The radiation is invisible; you need a detector to find it.

Where it leads