Investigating patterns of similar chemical behavior in elements led to the Modern Periodic Law. When the first periodic table was published in 1869, the known elements were listed in order according to mass. This arrangement contained some discrepancies in the repeating patterns that were observed.
In the early 1900s, the periodic table was rearranged according to increasing atomic number, based on the number of protons in the nucleus of each atom. This modern arrangement eliminated problems seen on the earlier periodic table. Elements with similar properties were easily grouped in columns, or families, on the periodic table.
Source: Image by Wolfram Burner/creative commons via flickr