Subject knowledge and
challenging concepts
- Radioactive emissions are ionising, in other words they have the potential to turn other atoms into ions, which may represent a hazard to life.
- Some stable elements have unstable isotopes. The unstable isotope is identical to the stable isotope in every way, except that it has a different number of neutrons (difficult to detect as they are uncharged) in the nucleus of the atom. This affects its mass and makes it unstable. For example, Carbon 12 is stable but Carbon 14, which has two additional neutrons in its nucleus, is unstable and will decay. As it decays its level of activity decreases, so by measuring the level of activity, the age of the sample can be calculated. It is impossible to predict when a particular atom will decay; it is a random process. However, a particular sample of a radioactive element will become less active as time goes by, as there will be fewer atoms left that haven�t decayed.
- Unstable elements will decay into other elements with the emission of alpha particles, beta particles, or gamma rays. The new element formed may or may not be radioactive, depending upon what the previous element was. If it is radioactive, it too will decay, so there may be a whole sequence. This is explained in more detail at www.practicalphysics.org > Atoms and nuclei > Developing a model of the atom.
- The decay of Carbon 14 can be represented by a nuclear equation: 146C → 147N + 0 -1e
This is beta decay and it can be seen that the products are nitrogen (same number of nucleons but one more proton) and an electron. - Radioactivity can be measured by count rate. This is the number of radioactive emissions in a set period of time and is measured using a Geiger-M�ller (G-M) tube. The time taken for the count rate to halve is the half-life and for a particular element this is a constant, irrespective of the mass of the element or how long it has been decaying for. If, for example, the half-life of an element is 14.2 days, its level of radioactivity will always halve in that amount of time.
- This constant rate of decay can be used in calculating the age of certain artefacts. All living material contains carbon, and as the half-life of Carbon 14 is just over 5700 years, this is very useful for determining the age of artefacts from, for example, early human settlements. As different radioactive elements have significantly different half-lives, some are useful in dating rocks and determining the age of the Earth and the solar system, whilst having quite different uses. For example:
Table 1. Half-lives of some typical radioisotopes
| Radioisotope | Half-life |
|---|---|
| Polonium-215 | 0.0018 seconds |
| Bismuth-212 | 60.5 seconds |
| Sodium-24 | 15 hours |
| Iodine-131 | 8.07 days |
| Cobalt-60 | 5.26 years |
| Radium-226 | 1600 years |
| Uranium-238 | 4.5 billion years |
- When an atom decays it will emit one of the following:
- an alpha particle, which consists of two protons and two neutrons and is therefore a helium nucleus. This is comparatively massive; although its range is restricted, it is strongly ionising
- a beta particle, which is an electron. Its mass is around 8000 times less than an alpha particle; it is more penetrating but less ionising
- a gamma ray, which has no mass but is electromagnetic radiation, like light and infrared, but with a very short wavelength. It is more penetrating but less ionising than the particles described
- a positron, which is an electron with a positive charge. It is the antiparticle for an electron, and if they collide, each wipes the other out
- a neutron, which has no charge and a mass slightly greater than a proton.
- A G-M tube detects radioactive decay as the emission causes ionisation in the tube. Each count represents one atom that has decayed.
- It is possible to plot a graph showing the number of neutrons against the number of protons for stable elements. From this it can be seen that isotopes not lying on this curve are unstable and will decay.
- Nuclei with more than 82 protons usually undergo alpha decay, which involves releasing a helium nucleus. Its loss reduces the mass number by four and the atomic number by two.
- Isotopes above the curve will emit beta radiation: one of the neutrons has split into a proton and an electron, and the electron has been released. The mass number will be the same, but the atomic number will have been increased by one due to the additional proton.
- Isotopes below the curve will emit beta+ radiation: one of the protons has split into a neutron and a positron, and the positron has been released. The mass number will be the same, but the atomic number will have been decreased by one due to the lost proton.
- Either kind of beta decay often results in a rearrangement of the nuclei with the loss of energy as gamma radiation. There is no change to the mass or atomic numbers.