Periodic Classification of Elements

Dmitri Ivanovitch Mendeleev (1834-1907), who believed that the properties of chemical elements are not the result of chance, and that they vary periodically according to the atomic mass of the elements represented, described a table between 1869 and 1871 comprising eight columns, in which elements with similar properties were grouped in order of increasing atomic mass from top to bottom, and nineteen rows, in which the elements were arranged in order of increasing atomic mass from left to right. The arrangement meant that only elements with similar chemical properties were listed in the same column.

This periodic classification is different from the one used today, but similar in principle: it proposes a systematic classification of chemical elements closely linked to the periodicity of their chemical properties.

Since the end of the 19th century, the periodic table has undergone numerous readjustments, with the addition of natural elements unknown at the time of Mendeleïev – but which Mendeleïev had foreseen by leaving empty cells in his table –, and artificial elements, until it took on the form we know today. This is a universal classification, which has been enriched with physical data and to which all types of physical and chemical behavior of the elements can be related. Currently, its standard form comprises 118 elements, ranging from ₁ H to ₁₁₈ Uuo.

The table of elements contains various physical and chemical quantities that are characteristic of the element. These quantities are reference values, i.e. they have been standardized by various organizations, such as the International Union of Pure and Applied Chemistry (IUPAC), the National Bureau of Standards (NBS) or the Committee on Data for Science and Technology (CODATA). These quantities are described in detail below.