Nuclei often behave like magnets through a property called spin. When placed in a (strong) magnetic field, these magnets 'line up' either with or against the external field. A small energy difference, one depending upon many factors but especially the strength of the imposed external field, will prevail for the two states. When photons (electromagnetic radiation) of just the right energy (radio frequencies which are lower in energy than the visible light) are supplied, the nuclei can absorb the energy and change orientations. When this occurs, a decrease in the total number of photons often can be measured. The is the basis of the nuclear magnetic resonance experiment.

Because the electrons in molecules also behave like magnets, they too interact with the magnetic field. As a result, the energy difference is a function of the electronic environs of the nucleus. This is the basis an important molecular structure tool, nuclear magnetic resonance. Two of the most important nuclei studied are H-1 and C-13. Also occasionally studied are P-31 and F-19.

Discussion of these experiments is deferred to other courses in this series. However, this property gives rise to magnetic resonance imaging, discussed in this course.