Among the first and most important contributions to chemistry was the periodic table, which arranges the chemical elements according to their increasing atomic number. This arrangement has a recurrent pattern, called the periodic law, which is the basis for a wide range of chemical relationships among the elements.

The first such chart was published in 1862 by French geologist A.-E.-B. de Chancourtois, who transcribed the atomic weights of the elements and plotted them on the surface of a cylinder with a circumference of 16 mass units (corresponding to their approximate atomic weights).

This helical curve brought closely related elements onto corresponding points above or below one another in the chart. Eventually, this idea was confirmed by Ernest Rutherford’s studies of the scattering of alpha particles by heavy atom nuclei.

It was in this context that he suggested the use of the atomic number to order the elements. It is still used today, although the precise relationship between the atomic number and valence has not yet been clearly established.

However, the exact atomic weight of an element is of very little significance for its position in the periodic system. Rather, its valency plays an important part in the periodic law. This is demonstrated by the existence of isotopes of every element-atoms that have a different atomic weight but share their chemical properties.