An early version of the Periodic Table had the elements listed in order of increasing relative atomic mass instead of increasing atomic number. Doing so today would result in the reordering of three
pairs of elements in the first five periods. Identify the three element-pairs.

Although he was unaware ofit, Mendeleev had actually placed the elements inorder of increasing “atomic number,” a numberrepresenting the amount of positively charged protons in the atom (also thenumber of negatively charged electronsthat orbit the atom). Mendeleev went even further.