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

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

3. 3. Jöns Jakob Berzelius 1779–1848 1828 Developed a table of atomic weights Introduced letters to symbolize elements

4. 4. Johann Döbereiner 1780-1849 1829 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

6. 5. A.E.Beguyer de Chancourtois 1820-1886 Credit for the first periodic table (published in 1862) probably should be given to a French geologist, De Chancourtois.

7. 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.

9. 6. John Newlands 1837 -1898 1864 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'.

11. 7. Lothar Meyer 1830–1895 1869 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

13. 8. Dmitri Mendeleev 1834 - 1907 1869 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.

16. 9. William Ramsay 1852 - 1916 1894 Discovered the Noble Gases

17. 10. Ernest Rutherford 1871 - 1937 1911 announced the nuclear model of the atom.

18. 11. Henry Moseley 1887-1915 1914 Determined the atomic number of each of the elements.

19. 12. Glenn Seaborg 1912 – 1999 1940 Synthesised transuranic elements (the elements after uranium in the periodic table)