Until fairly recently most people thought that everything was made of Earth, Air, Wind and Fire. But due to the hard work of some dilligent scientists we have discovered atoms. So let's take a quick look at this gang of heroes!
John Dalton was one of those all-rounders, he made contributions to many areas of science. His biggest achievement was the development of atomic theory, he came up with five "key ideas":
In 1897 J.J. Thomson announced an amazing discovery - atoms were made of smaller particles. Through his experiments he had discovered electrons, he determined that they i) have a negative charge and ii) they're far smaller than an atom .
He came up with a model/theory to explain his observation. Atoms consists of a cloud of positive charge with negatively charged electrons embedded. This idea is called the Plum Pudding model.
These three chaps were pretty much playing around in the lab from 1909-1911 when they broke Thomson's Plum Pudding model by discovering that the positive charge in an atom was concentrated in the atom's centre. They named this ball of positive charge the nucleus.
They then refined Thomson's model to create the nuclear model of the atom - this is shown in the picture to the right. This shows a key principle in science - a model is developed that fits the available data, when new data is discovered that contradicts the current model it is adapted or discarded for a better one. Rutherford took all the credit even though his students did the work......
Bohr was epic. He's the Thor of science. He took Rutherford's model, in 1913, and adapted it to explain flame emission spectra (there's a page on this in the Trilogy section if you want to know what it is). He determined that electrons move around the nucleus at fixed distances.
This model is shown in the diagram to the left - compare it to the diagram above. Notice how the electrons are now held at fixed distances from the nucleus, we've met this idea before - they're called electron shells.
Lots of scientists tried to make mathematical models to explain the behaviour of the nucleus (this is an A-level physics concept, it's to do with something called spin and how mass affects it) and no-one could come up with something that worked. Atoms seemed to be about twice as heavy as they should be and many scientists were starting to think that there may be another particle involved. Proof finally came when James Chadwick managed to prove neutrons existed and determine their mass in the early 1930s. It also soon became clear that they had been difficult to discover as they're neutral, so they don't interact with protons and electrons.
Each time a particle was discovered our model of the atom was refined to best fit the available experimental evidence. Best not to get too attached to an idea in science - a new one will be along soon!