The law of octaves in chemistry is a generalization of a correspondence between chemical properties and atomic weight, in which different elements with similar chemical properties are usually separated by some multiple of eight mass units. It was first stated by the English chemist John Newlands in 1865.
John Newlands grouped the 56 known elements of the time into 11 different groups, then noted where two different elements had very similar chemical properties. He was the first person to recognize the periodic nature of the elements, and his insights were a precursor to the later periodic law. His contribution to the understanding of chemistry was not recognized until many years later. The first person to actually create a periodic table similar to the one used today was Dmitri Mendeleev. He first presented the table to the Russian Chemical Society and a Russian journal in 1869. His periodic table enabled him to predict the existence of undiscovered elements such as germanium, gallium and scandium, which he represented by blank spaces.
These early understandings of elemental periodicity were based on atomic weight. This created some errors, since there are elements with greater atomic weights, but lower atomic numbers, than adjacent elements. It was not until 1914 that the true sequence was discovered with the use of X-rays.