• # Jakob Berzelius

Jakob Berzelius developed a table of atomic weights.
• # Johann Dobereiner

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.
• # Alexandre Emile Beguyer de Chancourtois

Published the first list of elements that was ordered by increasing atomic mass. Elements with similar properties went on the same row. His work was ignored until Mendeleev.
• # John Newlands

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.
• # Dmitri Meldeleev

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.
• # Dmitri Mendeleev

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.
• # Lord Rayleigh, William Ramsey

Argon is discovered. It was chemically inert, not fitting into any of the groups in Mendeleev's periodic table. William Ramsey discovered the rest of the inert gases and put them in Mendeleev's periodic table. (Inert gases are now called the noble gases.)
• # Ernest Rutherford

Rutherford discovers protons.
• # Henry Moseley

He subjected known elements to x-rays and found a relationship between x-ray frequencies and number of protons. Moseley arranged elements by atomic numbers, not mass, which removed some of the inconsistencies with Mendeleev's table.
• # Glenn Seaborg

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