Periodic Table Evolution

He wrote the first extensive list of elements containing 33 elements & distinguished between metals and non-metals

He developed a table of atomic weights & introduced letters to symbolize elements

He proposed the Law of Triads: the middle element in certain triads had an atomic weight that was the average of the other two members. He began grouping elements based on similarities. He discovered that calcium (atomic weight 40), strontium (atomic weight 88) and barium (atomic weight 137) had similar properties, and that strontium had an atomic weight halfway between calcium and barium. He discovered it was the same for lithium, sodium, and potassium.

He ordered 62 known elements in order of increasing atomic weight. He noticed that after eight elements occurred, the ninth element displayed similar properties to the first one. He wrote a paper saying that each element showed similar properties to the eighth element following it. He created the Law of Octaves: Elements exhibit similar behaviour to the eighth element following it in the table.

He develops an early version of the periodic table, with 28 elements organized by valence

Ordered elements by atomic weights and properties (Lothar Meyer independently reached a similar conclusion, but his was published after Mendeleev's). The table contained 17 columns, two partial periods with 7 elements in each and two nearly complete periods.

He changed the table, giving it 8 columns. This table showed diagonal, vertical and horizontal relationships between elements. It contained gaps where Mendeleev predicted new elements would be discovered. His Periodic Table included the 66 known elements organized by atomic weights.

He discovered the Noble Gasses

He determined the atomic number of each of the elements and modified the 'Periodic Law'. Moseley arranged elements by atomic numbers, not mass, which removed some of the inconsistencies with Mendeleev's table.

He experimented with plutonium and discovered the transuranium elements, numbers 94-102 and changed the periodic table by putting them at the bottom.