ABSTRACT
Each chemical element has a unique number of protons and electrons (see 3 and 4), and the electrons are arranged in a particular way within shells (see 7). However, while each element is unique, groups of elements do share aspects of their general behaviour. This applies particularly to the way that they react with other elements. The shared behaviour is owing largely to similarities in electron configuration, and not surprisingly it is the outer, or valence, shell which is the key feature. From the mid-nineteenth century, some chemists attempted to design schemes which presented the elements in a way which reflected their similar properties. The most notable of these schemes was proposed by the Russian chemist Dmitri Mendeleev (sometimes spelt Mendeleyev) in 1869. It was given the name periodic table. In his scheme the elements were arranged in columns in terms of increasing atomic weight (atomic particles had yet to be discovered so atomic numbers were unknown). When the table was proposed, only about sixty elements were known, but Mendeleev correctly predicted the existence of other elements, their place in the scheme, and their properties. Although Mendeleev’s scheme has been refined with the discovery of other elements, and some elements have been moved to a new position, the basic structure has endured. A modern periodic table of the elements is shown in Figure 15.1.
