7.1-7.3 - Radioactivity units, atoms and isotopes

Using radioactivity units

The radioactivity section uses several units that you must be able to recognise and use correctly.

The becquerel is the unit used for activity. At this level, $1\text{ Bq}$ means one nuclear decay per second. A source with a larger activity has more decays each second, but this lesson does not yet compare radiation types or calculate half-lives.

Time units are often mixed in questions, so convert them before comparing values:

For length, $1\text{ cm} = 0.01\text{ m}$. In this topic, centimetres are useful when describing small distances, such as absorber thickness or detector position in later practical work. Always write unit symbols exactly: Bq, cm, h, min and s.

Inside an atom

An atom has a tiny central nucleus with electrons around it. The nucleus contains protons and neutrons. The electrons are outside the nucleus.

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The nucleus is small compared with the whole atom, but it contains nearly all of the atom's mass because protons and neutrons are much more massive than electrons. In radioactivity, most changes involve the nucleus, so you need to be fluent with the words proton, neutron, electron and nucleus.

Do not describe an atom as only a solid ball. Also do not put electrons inside the nucleus: electrons are outside the nucleus.

Reading nuclear symbols

A particular nucleus can be described using a chemical symbol with two numbers beside it. The general form is:

Here, $\text{X}$ is the chemical symbol for the element, $A$ is the mass number, and $Z$ is the atomic number. For example:

This symbol means a carbon nucleus with mass number 14 and atomic number 6. The C tells you the element is carbon. The lower number, 6, tells you the number of protons in the nucleus. The upper number, 14, tells you the total number of protons and neutrons in the nucleus.