Development of the periodic table

Dalton (1803) arranged known elements in order of atomic weights. Newlands (1865) noticed that every eighth element, in order of atomic mass, repeated similar properties. Mendeleev swapped the order of atomic weights in order to fit elements better into groups of similar properties. Mendeleev’s periodic table had gaps and he predicted missing elements that were later found. Mendeleev was not aware of isotopes, so was unable to explain why the ordering by atomic weight was not always correct.