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Atoms go pear-shaped

By Pat, 30 August 2013 News

Diagram showing pear-shaped nucleus.

The shape of Ra-224, as worked out from CERN measurements.
Image: CERN

Atoms – we know that they’re small. But what do they actually look like? Recent experiments suggest that some atoms have a surprising shape.

To understand what an atom might look like, we need to know what they’re made of. An atom has a nucleus in the centre, which contains most of the atom’s mass. A distance away from the nucleus are small (even by atomic standards), negatively charged particles called electrons.

The size of the nucleus varies, depending on the type of atom. Hydrogen has the simplest nucleus, consisting of a single, positively charged particle called a proton. Every atom other than hydrogen has a nucleus made of different combinations of protons and similarly-sized particles called neutrons. As their names suggests, neutrons have no charge.

Atomic nuclei can contain well over 100 protons and over 100 neutrons. Protons have a positive electric charge, so they repel each other. Yet many nuclei are stable. How do they stick together? There is a force, called the strong force that attracts protons and neutrons. This force overrides the repulsion of the protons and holds the nucleus together.

You might have seen pictures of atoms that show little spheres clumped together at the centre, with electrons whizzing around in circular orbits around the outside. However, this isn’t really accurate. While it’s useful to think of protons and neutrons as particles, this doesn’t mean they’re like tiny balls. The strong force pulls them together extremely tightly, and they’re also rearranging and changing shape.

Physicists can determine the shape of nuclei by stripping away their electrons and accelerating the nucleus to a high speed. The nucleus passes through a thin metal foil, creating gamma radiation. By analysing the radiation, it is possible to make conclusions about the shape of the nucleus.

Most nuclei have an oval shape like that of a rugby ball, which is predicted by current theories. Physicists in Europe have shown that two nuclei, radium-226 and radon-220, have a less-symmetric pear shape. Such a shape is not predicted according to the theory.

If these observations are accurate, the theory will need to be adjusted to accommodate these new, fruity flavours of nuclei.

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