Development Of The Periodic Table

Question 1

Early 1800s

Answer 1

Arranged by atomic weight

Question 2

Dmitri Mendeleev 1869

Answer 2

Put elements in order of atomic weight but switched that order if the properties meant that it should be changed. He left gaps to make sure that the elements with similar properties stayed in the same groups. These gaps indicated the existence of undiscovered elements and Mendeleev could predict their properties