04. Nuclear stability

Outcomes:

> be able to state/explain why isotopes exist.

> to describe and explain the existence of a specific isotope

> to research and communicate an example of scientific and technological innovation that uses isotopes

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^Binding energy curve. This graph on atomic nuclei binding energies per nucleon plotted against the number of nucleons for the first 94 chemical elements. Lighter elements undergo fusion to become more stable, whereas heavier elements undergo nuclear fission.

Binding Energy: Nuclear binding energy is the energy required to separate a nucleus into its constituent parts, namely protons and neutrons.

An implication of the short-range nature of the strong nuclear force is that the actual configuration of nucleons can make a difference to the stability of the atom. In 1913, the British physicist J.J. Thompson (1856 - 1940) was using electrical fields to deflect different types of atom, and he noticed while experimenting with neon that two deflections could be seen on the fluorescent screen of his apparatus.

Thompson had discovered the existence of isotopes. An isotope is a variation on an element: while all isotopes of an element have the same proton number (by definition), they have different numbers of neutrons. Even hydrogen - the simplest possible element - has two other known isotopes.

Some isotopes are more stable than others. The most stable forms tend to be more common, while less stable forms may decay into other substances. In reality, whenever we gather a sample of a particular element, we will inevitably have a mixture of different isotopes of that element.

These discoveries suggested that the nucleus was neither a solid, nor as simple as had been thought. It is the inherent instability of larger nuclei and some isotopes that is responsible for the mysterious rays that Roentgen and Becquerel noticed. We now know that these rays are, in most cases, not rays at all, but small particles of matter that are ejected from nuclei when they change to a more stable state.

Activity:

Split an A3 piece of paper in half:

• • On the left hand side use the information from the first 4 'lessons' in this unit to explain why and how isotopes exist.

  • On the right hand side, research and report about a scientific/technological use of an isotope and include an explanation of why the isotope is used as opposed to the 'standard atom'.