Scientists discovered that when they arranged elements in order of increasing atomic mass there is a repeating pattern of properties (A).The Periodic law was developed by Dmitri Mendeleev and Lothar Mayer in 1869. They worked independently of each other and both created a periodic table. They also both arranged the elements on the table by weight and they both suggested that certain properties of the elements reoccur. Mendeleev's table is the most important table because it the values for atomic mass are the most accurate.