Periodicity

Periodicity is the regular repetition of chemical and physical properties of the elements when listed by atomic number or (with two or three exceptions) by atomic weight. It forms the basis of the periodic table.

Periodicity was discovered in the 1860s by English chemist John Newlands and (independently) by French geologist Alexandre-Emile Béguyer de Chancourtois, and was fully developed by Russian chemist Dmitri Mendeleev in his seminal texbook of inorganic chemistry (1869). It was not explained until 1925, when Wolfgang Pauli correctly described the rules governing electron configuration.

In its modern form, periodicity leads to eighteen groups of elements, which are represented as columns in the periodic table. The chemistry of the elements in any given group shows obvious similarities, but also systematic trends as one descends the group from top to bottom.