Chemistry - The Periodic Table (AQA)

A French scientist, Antoine Lavoisier, gathered together a list of everything that he believed to be an element in 1789 - but in no particular order. An element was defined as a substance that could not be broken down further so some of his elements were in fact compounds. Following the discovery of electricity the British scientist, Humphrey Davy, experimented with passing it through the elements on Lavoisier's list. In doing this he found that some of them could in fact be broken down into simpler substances, the list of true elements increased.

When scientists learnt to measure the atomic masses of elements, they started to use this property to arrange the elements. A German chemist, Johann Döbereiner, spotted that there were groups of three elements that seemed quite similar in their properties and noted that the atomic mass of the middle one was the average of the other two. He had spotted the start of several of the periodic table groups.

A British scientist, John Newlands, took it further. He arranged the elements in order of atomic mass and noticed that every eighth element seemed to behave in a similar way. Finally, the key step was taken by a Russian, Dimitri Mendeleev. His ideas were similar to Newlands' and he arranged the elements in order of their atomic mass. The difference was that he left gaps where he thought that elements had not been discovered. When the first of these 'missing elements' was found and seen to have the properties Mendeleev predicted, scientists in the world of chemistry knew he was onto something.

Since then, scientists have started to understand atomic structure. The modern periodic table is arranged in order of atomic number. We now know that it is the electrons and how they are arranged that give the elements their chemical properties.