What was Henry Moseley's contribution to the development of the periodic table?
Mendeleev's original table had the elements arranged in order of increasing atomic mass . Moseley organized his table in order of increasing atomic number . While atomic mass and atomic number generally correlate, because some elements have more, neutron heavy isotopes than others, they can have a higher atomic mass despite having a lower atomic number.
Examples of this include:
Number 27, cobalt, being more massive than number 28, nickel
Number 52, tellurium, being more massive than number 53, iodine
Number 92, uranium, being more massive than number 93, neptunium