1. Known Elements in 1800

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

3. 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

4. Compare Older Tables with the modern table

5. 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