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Not all atoms of an element are identical. Although every atom of a given element has the same number of protons, the number of neutrons in the nucleus can vary, giving atoms of the same element with different masses — these are called isotopes. Some isotopes have nuclei that are perfectly stable and last forever; others have nuclei that are unstable and break apart over time, throwing out energy and particles in a process called radioactive decay. This lesson, part of Topic P6 (Radioactivity) of OCR Gateway Science A, defines isotopes and explains why some nuclei are unstable, introduces radioactive decay as a random process, defines activity in becquerel, describes how radiation is detected, and surveys the background radiation that surrounds us all the time.
By the end of this lesson you should be able to define an isotope and write isotopes using nuclide notation, explain why some nuclei are unstable, describe radioactive decay as a random process, define activity and its unit the becquerel, describe how a Geiger–Müller tube detects radiation, and list the main sources of background radiation.
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have the same number of protons (the same atomic number Z), they are the same element with the same chemical behaviour; but because they have different numbers of neutrons, they have different mass numbers A.
A classic example is carbon, which has three naturally relevant isotopes, all with 6 protons:
| Isotope | Protons | Neutrons | Mass number | Stable? |
|---|---|---|---|---|
| 612C (carbon-12) | 6 | 6 | 12 | Stable |
| 613C (carbon-13) | 6 | 7 | 13 | Stable |
| 614C (carbon-14) | 6 | 8 | 14 | Unstable (radioactive) |
All three are carbon — they all have 6 protons — but carbon-14, with two extra neutrons compared with carbon-12, has an unstable nucleus and is radioactive. Hydrogen provides another example: ordinary hydrogen 11H (1 proton, 0 neutrons), deuterium 12H (1 proton, 1 neutron), and tritium 13H (1 proton, 2 neutrons, radioactive).
Exam Tip: Isotopes have the same number of protons (so same element, same atomic number) but a different number of neutrons (so different mass number). A reliable one-line answer: "Isotopes are atoms of the same element with the same proton number but different numbers of neutrons."
Inside a nucleus, positively charged protons are crammed extremely close together, and like charges repel strongly. Holding them together is a powerful short-range attraction called the strong nuclear force, which acts between nucleons. The neutrons play a vital role here: they add to the strong-force attraction without adding any repulsion (they are uncharged), helping to "glue" the nucleus together.
A nucleus is stable when the forces inside it are properly balanced — there is the right ratio of neutrons to protons. A nucleus becomes unstable when this balance is upset, for example when it has:
An unstable nucleus has too much energy and is, in effect, the wrong shape or composition to last. To become more stable, it decays — it emits a particle and/or energy as radiation, changing its make-up so that it moves towards a stable arrangement. This emission of radiation from an unstable nucleus is what we mean by radioactivity.
Exam Tip: Link instability to the balance of neutrons and protons. An unstable nucleus has an unfavourable neutron-to-proton ratio (or is simply too big), so it has too much energy and decays by emitting radiation to become more stable.
A crucial idea in this topic is that radioactive decay is a completely random process. This has two parts:
Radioactive decay is also spontaneous: it is not triggered by anything outside the nucleus. Heating the sample, cooling it, crushing it or reacting it chemically makes no difference to whether or when a nucleus decays — the decay comes entirely from within the unstable nucleus itself.
Even though we cannot predict a single nucleus, a large sample contains an enormous number of nuclei, and on average a fixed fraction of them decays in each second. This is why radioactivity is predictable on average (which is what makes half-life, in a later lesson, a reliable measure) even though each individual decay is random. The situation is rather like rolling a huge number of dice: you cannot say which die will land on a six, but you can say very accurately that about one-sixth of them will.
Exam Tip: "Random" means you cannot predict which nucleus will decay or when. "Spontaneous" means the decay is not affected by temperature, pressure or chemical changes. Both phrases score marks — examiners often ask you to explain what "random" means.
The activity of a radioactive source is the rate at which nuclei in the source decay — in other words, the number of nuclear decays each second. Activity is measured in becquerel (Bq), where:
1 Bq=1 decay per second
A source with an activity of 500 Bq has, on average, 500 of its nuclei decaying every second. Because real sources can have very large activities, you will often meet kilobecquerel (1 kBq=1000 Bq) and megabecquerel (1 MBq=1000000 Bq).
As a source decays, the number of undecayed unstable nuclei left in it falls, so there are fewer nuclei available to decay, and the activity decreases over time. A freshly prepared source is more active than the same source weeks or years later — a behaviour governed by its half-life.
The count-rate is the closely related quantity that we actually measure: it is the number of decays detected per second by a radiation detector. The count-rate is always a bit lower than the true activity, because a detector cannot capture every single particle — some radiation misses the detector or is absorbed before reaching it. Even so, the count-rate is proportional to the activity and is what we plot and use in experiments.
Exam Tip: Activity is the number of decays per second, measured in becquerel (Bq): 1 Bq=1 decay per second. Activity falls over time as fewer unstable nuclei remain. Do not confuse activity (true decay rate) with count-rate (the rate actually detected).
We cannot see, hear, smell or feel nuclear radiation, so we need an instrument to detect it. The most common detector is the Geiger–Müller (GM) tube, usually connected to a counter or ratemeter.
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