What are atoms?

The idea of an atom arises from a simple question: is it possible to keep breaking a piece of matter into smaller and smaller pieces or do you eventually get to the smallest piece of matter that cannot be divided any further?

It turns out that the breaking cannot go on forever and there is a smallest piece of matter that retains the overall properties of that substance: we call this an atom. The word “atom” comes from the Greek word “atomos”, which means indivisible (uncuttable). This description works for chemistry but it isn’t strictly correct because atoms can indeed be broken into smaller pieces – and that’s what the Radioactivity module is all about!

When it comes to visualising atoms, there are four models that we need to know about;

• small solid lumps – as imagined by Democritus around 400 BC and revived by John Dalton in the early 1800s. Although the lumps were given different shapes and features, such as “hooks” for holding on to adjacent atoms, we usually refer to the lumps as being smooth spheres (like snooker balls).

• the “plum pudding” model – proposed by J J Thomson in 1904 and comprising tiny negatively charged particles embedded in a positively charged matrix.

• the separate nucleus model – proposed by Ernest Rutherford in 1911. This divided the atom into a tiny, dense and positively charged nucleus at the centre with negatively charged electrons distributed around the outside. The evidence for this model came from Rutherford’s gold foil experiment, which is explained in detail on p89 of the CGP Complete Revision and Practice book.

• electron shells – proposed by Neils Bohr in 1913. This confined the orbiting electrons to discrete shells and introduced the idea of electron energy levels. These energy levels are used in lasers and explain the atomic spectra that can be used to measure the expansion of the Universe (a useful learning link to knowledge you are expected to have in the Astronomy module).

You may notice that the last three developments all took place within a period of just 10 years. You may also notice that the models are based on positive and negative charges, with no mention of the neutron. That is because the neutron, which will turn out to be an essential requirement for understanding radioactivity, wasn’t discovered until 1932. Despite the small delay, there was a direct link back to the first of the modern models; James Chadwick, who discovered the neutron, studied under Ernest Rutherford, who in turn studied under J J Thompson – all at Cambridge University.

Andy Brunning’s excellent Compound Interest website has a great visual summary of the various models of the atom: you can find out more here and also download a mini-poster to refer to as part of your revision.