Known Elements in 1800

Mendeleev 1869…… He listed the elements in increasing atomic mass. and placed elements with similar chemical and physical properties in vertical groups

Difference between mendeleev’s table and the modern table He left gaps for as yet undiscovered elements Nobel gasses not on the table Transition elements not in a separate block Mendeleev arranged the elements by increasing atomic weight The modern table arranges the elements by increasing atomic number

Compare Older Tables with the modern table

Mendeleev’s problem with Te and I When Mendeleev arranged by increasing atomic number, Te and I ended up in a group of elements with which they are not similar chemically Se Br I 126.92 52 Te 127.61 53 Sb 121.76 Xe 131.3 At Po Mendeleev reversed the order of Te and I by weight on his table so that they would be in a group with similar properties Moseley showed that if the elements are be arranged by increasing Atomic number, then Tellurium correctly came before Iodine