Rearranging the Periodic Table of Elements

In 1912, Neils Bohr suggested that arranging the elements on the periodic table by atomic number (the number of protons) rather than by atomic mass would solve several problems with the then current arrangement of the elements on the table.

This was solution to a problem that had persisted since Mendeleev's table was developed in the late 1880's -- specifically the "wrong" arrangement of several elements. For instance if you look at Nickel and Cobalt on the periodic table, you will notice that they are in the "wrong" order by mass (cobalt weighs slightly less than nickel). But, if you order them by mass, their properties no longer fall into the periodic trends properly.