3a1 History of the Periodic Table

History of the Periodic Table.

Newlands arranged the elements in order of atomic weight and identified that every eighth element had similar properties. This doesn’t quite work on the modern periodic table because we have now got the noble gases too.

Mendeleev did the same kind of thing but arranged the elements so that the “similar elements” were in vertical groups. His main claim to fame was that he left gaps where elements had not yet been discovered. The most famous example is the gap left for what we call Germanium (he called it eka-silicon). Chemists were pleased to discover the element had the properties that Mendeleev had predicted.

Later, with the discovery of the details of atomic structure we can understand that it is better to arrange the elements in order of atomic number rather than in order of atomic mass. For most of the elements the order is the same but a major difference occurs with potassium and argon. If the elements were in order of mass, argon (noble gas, very unreactive) would be in the same group as sodium while potassium (a very reactive metal) would be in the noble gases. This problem is overcome when the order is based on the number of protons in the atom. Since this links to the number of electrons, which in turn links to the chemical properties, this is a more sensible arrangement.