1. Aristotle 384 – 322BC Four element theory - earth air fire water

Antoine Lavoisier 1743 – 1794 He considered 33 substances as "elements" "substances that chemical analyses had failed to break down into simpler entities."

3. Jöns Jakob Berzelius 1779– Developed a table of atomic weights Introduced letters to symbolize elements

4. Johann Döbereiner Developed 'triads', groups of 3 elements with similar properties. Lithium, sodium & potassium formed a triad. Calcium, strontium & barium formed a triad. Chlorine, bromine & iodine formed a triad. Forerunner to the notion of groups

5. A.E.Beguyer de Chancourtois Credit for the first periodic table (published in 1862) probably should be given to a French geologist, De Chancourtois.

A list of the elements positioned on a cylinder in terms of increasing atomic weight. The cylinder was constructed so that 16 mass units could be written on the cylinder per turn. Closely related elements were lined up vertically.

6. John Newlands The known elements (>60) were arranged in order of atomic weights and observed similarities between the first and ninth elements, the second and tenth elements etc. He proposed the 'Law of Octaves'.

7. Lothar Meyer 1830– Lothar Meyer compiled a Periodic Table of 56 elements based on the periodicity of properties such as molar volume when arranged in order of atomic weight

8. Dmitri Mendeleev By arranging all of the 63 elements then known by their atomic weights, he managed to organize them into groups possessing similar properties. Where a gap existed in the table, he predicted a new element would one day be found and deduced its properties. Three of those elements were found during his lifetime: gallium, scandium, and germanium.

9. William Ramsay Discovered the Noble Gases

10. Ernest Rutherford announced the nuclear model of the atom.

11. Henry Moseley Determined the atomic number of each of the elements.

12. Glenn Seaborg 1912 – Synthesised transuranic elements (the elements after uranium in the periodic table)