3. Splitting the atom page 11
Stable isotopes
Every element has a number of different isotopes. For example, lithium has isotopes lithium-6, lithium-7 and lithium-8 (see page 9). The first two of these are stable. However, lithium-8 is unstable - it will decay into a different element by giving out beta radiation.

All the elements have radioactive isotopes and most of them (up to Z = 82) have at least one stable isotope.

Interactive atom becoming an ion
Picture 3.1 Graph of stable isotopes.
Plotting a graph

We can plot a graph of stable isotopes. The number of protons (or atomic number, Z) is on the x-axis and the number of neutrons (N) is on the y-axis. The graph is a curve - see picture 3.1.

For light elements (the bottom left of the graph), the stable isotopes are the ones with the same numbers of protons and neutrons and hence the ‘line of stability’ follows the straight line of N = Z. For example, the stable isotope carbon-12 has 6 protons and 6 neutrons.

Heavier elements
Stable isotopes of the heavier elements (top right of the graph) have more neutrons than protons. For example, Gold-197 is stable. It contains 79 protons and 118 neutrons.

Hence the line begins to curve upwards, away from N = Z.

Nuclear glue
The neutrons in a nucleus can be thought of as acting as a kind of glue to hold the nucleus together. The positively charged protons are in a very confined space and would rather not be, since they repel each other by the electromagnetic interaction.

However, protons and neutrons are all attracted to each other due to another force (the strong nuclear force), and hence having more neutrons around can help to hold the nucleus together. Notice that no amount of neutrons can hold a nucleus together once it has more that 82 protons – the line stops at Z = 82! All of the elements with an atomic number greater than 82 have only unstable isotopes.

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Question 10
Look at the isotopes below. Some of them are stable and some are unstable. First of all, fill in the number of protons and neutrons. Then decide which ones are stable and which are unstable.
isotope Z N