The periodic table is most useful in that study of an elementæs position allows the prediction of its properties. Knowing that elements arranged in order of increasing proton number demonstrate periodicity of properties - that is, they fall into groups distinguished by certain common characteristics - and knowing that certain properties may become more or less pronounced as we move down a group or along a period, it is possible to construct a general picture of an element, even if the element has never been studied individually.
If, for example, we know that sodium, in group 1 of the periodic table, is a reactive metal of low density, and low melting and boiling points, then we can predict that the other members of the same group will possess similar properties. The table is not an infallible guide - there are various irregularities - but as a general aid to predicting the chemistry of the elements it is extremely useful.
Dmitri Mendeleyev, the Russian chemist who formulated the periodic law, and produced tables that are the forerunners of the ones that we know today, used the periodic table to predict the discovery of ten elements, and predicted the properties, with great accuracy, of four of these. Seeing a gap in the table below silicon, for example, he predicted the existence of an element which he called eka-silicon, and which is now known as germanium.
Modern scientists have used the table in the same way, and have predicted the existence and properties of many transactinide elements. It is expected, for example, that the element with the proton number of 112, will be positioned below mercury in the periodic table, and will possess similar properties to the other members of group 12.
