Periodicity refers as  variations in element properties with increasing atomic number
In studying the characteristics of various elements, the early chemists started to list various properties. John Dalton devised a means to determine a relative mass for the atoms of the elements. This would later allow us to arrange the elements in a table. This table, initially set up by the Russian scientist Dmitri Mendeleev, is the single most important tool of chemists everywhere in the world. In about 1868, Mendeleev arranged the 60 elements he knew of at the time in order of each element's increasing mass. This showed him a repeating pattern of other characteristics of the elements. The tendency to show a regular repeating pattern is known as periodicity.