What did scientists discover when they arranged elements in order of increasing atomic mass?
a. a repeating pattern of properties
b. a puzzling series of numbers
c. alphabetical arrangement of the names of elements
d. no patterns or relationships

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.