Atoms and radioactivity, Year 10: nuclear equations, half-life and safety

Mass number 234, atomic number 90

HintTake away what the emitted particle carries off: its nucleons, and how many of them are protons.

The whyAn alpha particle is two protons and two neutrons, so it has mass number 4 and atomic number 2. Both numbers must balance on the two sides of the equation: 238 = 234 + 4 and 92 = 90 + 2. Alpha decay lowers both the mass and the charge of the nucleus.

Mass number 14, atomic number 7

HintA neutron has become a proton: count the nucleons before and after, then count the protons.

The whyIn beta decay a neutron turns into a proton. The total number of protons and neutrons is unchanged, so the mass number stays at 14, but there is one more proton, so the atomic number goes up by one. In the equation the beta particle is given mass number 0 and atomic number −1, so that 6 = 7 + (−1).

charge

HintIt is the property that the number of protons decides.

The whyA gamma ray is electromagnetic radiation, with no mass and no charge of its own. Emitting one carries energy away from the nucleus but leaves the number of protons and the number of neutrons exactly as they were, so in a nuclear equation the mass number and the atomic number stay the same.

Alpha

HintWork out what has gone missing from each of the two numbers.

The whyThe mass number has fallen by 4 and the atomic number by 2. Only one kind of emission carries away four nucleons, two of them protons. Beta decay would have left the mass number at 226, and gamma would have changed neither number.

4; 2

HintCount its protons and neutrons altogether, then its protons alone.

The whyAn alpha particle is made of two protons and two neutrons. The mass number is the total number of protons and neutrons; the atomic number is the number of protons. In the symbol the mass number is written at the top left of He and the atomic number at the bottom left.

The half-life of the isotope

HintThe name is two short words joined by a hyphen; the first is a fraction.

The whyIt is equally the time for the count-rate, or the activity, from the sample to fall to half its starting level. Each isotope has its own fixed value, from fractions of a second to billions of years, and nothing can speed it up or slow it down.

3

HintCount how many times the starting figure has to be halved to reach the final one.

The why800 halves to 400, and 400 halves to 200: that is two half-lives. Two half-lives take 6 hours, so one takes 6 ÷ 2. Working out a half-life from given information is something AQA expects at both tiers.

Sample A

HintA short half-life means the nuclei are in more of a hurry.

The whyHalf of A's nuclei will decay within two days, whereas B takes two thousand years to lose the same number. So far more of A's nuclei are decaying every second. A short half-life means a high activity that dies away quickly; a long half-life means a low activity that lasts.

A sample contains a huge number of nuclei, so the proportion that decays in a given time is predictable

HintYou cannot predict one toss of a coin, but you can predict roughly how many heads you get from a million.

The whyEach nucleus has the same fixed chance of decaying in each second. With an enormous number of nuclei those chances average out, so the same fraction decays in each equal interval of time. That steady fraction is what gives every isotope a definite half-life.

One sixteenth (1/16)

HintHalve, and halve again, once for each half-life.

The whyAfter one half-life the activity is ½ of the original, after two ¼, after three ⅛ and after four 1/16. As a ratio, final activity to initial activity is 1 : 16. Calculating the net decline as a ratio after a given number of half-lives is Higher tier content.

Radioactive contamination

HintThe same noun is used when dirt or germs get into food or water.

The whyThe hazard comes from the decay of the atoms that have been picked up, and how serious it is depends on the type of radiation they emit. Because the source travels with the object or person, exposure continues until the material is removed or has decayed.

Irradiation

HintIt begins with the same letters as 'irritate' and ends like 'radiation'.

The whyThe source stays outside the object, so the exposure stops as soon as the source is moved away or shielded. Medical instruments and some foods are deliberately treated this way to kill microbes.

radioactive

HintThe word describes a material whose own nuclei are decaying.

The whyIrradiation exposes an object to radiation from a source outside it. The radiation may damage or ionise the object's atoms, but it does not leave unstable nuclei behind. That is why irradiated fruit or sterilised surgical tools are safe to handle afterwards.

Inside, its strongly ionising radiation is absorbed by living cells; outside, it is stopped by the skin

HintRecall how far alpha particles travel, and what they do to the atoms they meet.

The whyAlpha particles cannot get through skin or even a few centimetres of air, so a source outside the body does little harm. Swallowed or breathed in, the same source sits next to living cells, which absorb all of its highly ionising radiation. Comparing the hazards of contamination and irradiation is something AQA expects.

Peer review

HintThe first word means 'an equal' — here, a fellow expert.

The whyStudies of how radiation affects people must be published and shared so that others in the field can examine the methods and evidence. Findings that survive this checking can be trusted and used to set safety limits. AQA names this process in the section on radiation hazards.