By the middle of the 19th century, a large number of chemical elements had been discovered, their atomic masses determined, and a large number of physical properties measured. The chemists of the time began to group the elements according to properties, giving rise to the first periodic tables of the elements. Periodic classifications allow us to see the similarities and differences between the different elements and predict properties of elements that were yet to be discovered.
In 1869, Dimitri Mendeleev and Lothar Meyer independently proposed periodic classifications of the elements, which are based on their organization in increasing order of atomic masses. This ordering grouped together elements with similar physical properties.