Radioactivity and half-lifeIB MYP Chemistry: Revision notes

Some nuclei are unstable, meaning they have too much energy or the wrong balance of protons and neutrons. An unstable nucleus becomes more stable by emitting radiation. This is radioactive decay, and it is random: you cannot predict when one nucleus will decay.

There are three types of radiation:

• Alpha (α): a helium nucleus, 2 protons and 2 neutrons, charge +2. Stopped by paper or a few cm of air. Strongly ionising.
• Beta (β): a fast electron from the nucleus, charge −1. Stopped by a few mm of aluminium. Moderately ionising.
• Gamma (γ): electromagnetic radiation with no mass or charge. Reduced by thick lead or concrete. Weakly ionising.

Ionising means the radiation knocks electrons off atoms, creating ions.

In a nuclear equation the mass numbers (top) and atomic numbers (bottom) must balance on both sides.

Alpha decay reduces the mass number by 4 and the atomic number by 2:

$^{238}_{92}\mathrm{U} \rightarrow {}^{234}_{90}\mathrm{Th} + {}^{4}_{2}\mathrm{He}$

Beta decay keeps the mass number the same and increases the atomic number by 1, because a neutron changes into a proton and an electron:

$^{234}_{90}\mathrm{Th} \rightarrow {}^{234}_{91}\mathrm{Pa} + {}^{0}_{-1}\mathrm{e}$

Check: 238 = 234 + 4 and 92 = 90 + 2. The nucleus that is left is a different element.

Background radiation is the low level of radiation that is always around us. Most of it is natural: radon gas from rocks and soil, cosmic rays from space, and radioactive carbon and potassium in food and our bodies. A small part is man-made, for example from medical X-rays.

Background radiation must be subtracted from the count rate when measuring a radioactive source.

The half-life is the time taken for the activity of a sample (the number of decays per second) to fall to half its starting value. It also equals the time for half of the radioactive nuclei in a sample to decay.

A decay curve is a graph of activity against time. It falls steeply at first, then more slowly, and never quite reaches zero. Each time interval equal to one half-life halves the activity.

Worked example: A sample has an activity of 80 counts per minute and a half-life of 3 days. What is its activity after 12 days?

Number of half-lives = 12 ÷ 3 = 4

80 → 40 → 20 → 10 → 5

The activity after 12 days is 5 counts per minute.

• Carbon dating: carbon-14 decays in dead wood and bones. Comparing its activity with living material gives the age.
• Medical tracers: a gamma emitter with a short half-life is injected, and a detector outside the body shows where it goes.
• Smoke detectors: an alpha source ionises the air. Smoke reduces the current and sets off the alarm.
• Sterilisation: gamma rays kill microorganisms on medical equipment and food, even through the packaging.

Ionising radiation can damage or kill cells, and can cause cancer or burns. Alpha sources are most dangerous if they are swallowed or breathed in, because the particles are strongly ionising. Safe handling includes:

• store sources in lead-lined containers
• use tongs or remote handling, and keep as far away as possible
• limit the time of exposure and wear dosimeter badges
• dispose of radioactive waste carefully and safely

Nuclear fission is the splitting of a large unstable nucleus, such as uranium-235, into smaller nuclei, releasing a lot of energy. It is used in nuclear power stations.

Nuclear fusion is the joining of two small nuclei, such as hydrogen, to make a larger nucleus, releasing even more energy. It powers the Sun and other stars.