In his invention of the Periodic Table of the Elements, Mendeleev had interchanged the orders of a few pairs of elements in order to put them in more appropriate places in this table of the elements.

Before Moseley's discovery, the atomic numbers (or elemental number) of an element had been thought of as a semi arbitrary sequential number, based on the sequence of atomic masses, but modified somewhat where chemists found this to be desirable, such as by the great Russian chemist, Dmitri Ivanovich Mendeleev.