1. Periodic Table

2. Lavoisier 1789 Traite Elementaire de Chimie. Produced the first table of elements Introduced a logical system for naming compounds and helped introduce the metric system May 8th 1794

3. Dalton, 1803, was the first chemist to use the term ‘atom’ He used this idea to explain how elements react together to form molecules. Dalton suggested that it should be possible to compare the masses of atoms. Hydrogen1 Carbone4.2 Oxygen5.5 Water6.5 Sulphur14.4 Sulphuric Acid25.4 Atomic Weights

4. Prout 1815 All atomic weights are multiples of Hydrogen and that Hydrogen is the fundamental element from which all other elements are formed. However, it was found that atomic weights are not whole numbers. E.g. Cl 35.46 More on atomic numbers

5. Dobëreiner 1817 Noticed that certain groups of 3 elements, e.g. Ca, Sr, Ba the atomic weight of 2 nd was approximately the mean the 1 st and 3 rd. Dobëreiner’s triads (5 in total) ElementRelative atomic mass Calcium40 Strontium88 Barium137 ElementRelative atomic mass Lithium7 Sodium23 Potassium39

6. Béguyer de Chancourtois 1862 He also realised that when elements are arranged in order of their atomic weight there was a repeating pattern of elements at regular intervals. That this happens when there are multiplies of 8x the atomic weight of hydrogen. So he arranged them in a spiral around a vertical cylinder divided into 16 vertical sections. Telluric Screw

7. Newlands ElementAtomic weights ElementAtomic Weights ElementAtomic Weights Hydrogen1Fluorine8Chlorine15 Lithium2Sodium9Potassium16 Beryllium3Magnesium10Calcium17 Boron4Aluminium11Chromium18 Carbon5Silicon12Titanium19 Nitrogen6Phosphorus13Manganese20 Oxygen7Sulphur14Iron21 Newlands in 1865, using Cannizzaro’s system (elements in order of succession) of atomic weights noticed a pattern, noticed that the 8 th one was a ‘kind of repetition of the 1 st. He called this the ‘Law of Octaves’. OK for the first 15 or so elements

8. Meyer Element I Atomic weights Element II Atomic weights Element III Atomic Weights B11Al27.3 C11.97Si28 N14.01P30.9 O15.96S31.98 F19.1Cl35.38 Li7.01Na22.99K39.04 ?Be9.3Mg23.9Ca39.9 Meyer in 1869, independently, put forward a similar list of elements. Meyer plotted an ‘atomic volume’ curve, showing that a quantitative property alternatively rises and falls over definite periods of the Elements.

9. Mendeleev 1871 Unlike Meyer, Mendeleyev believed in his convictions. In 1869 he published ‘Principles of Chemistry’ Mendeleev, without knowing about Meyer’s work, predicted as yet undiscovered elements. Meyer recognised Mendeleev’s work and both where awarded The Davy medal for Chemistry in 1882.

10. Mendeleyev 1871 periodIIIIIIIVVVIVII 1H 2LiBeBCNOF 3Na K Mg Ca Al * Si Ti PVPV S Cr Cl Mn Fe Co Ni 4Cu Rb Zn Sr *Y*Y * Zr As Nb Se Mo Br *Ru Rh Pd 5AgCdInSnSbTeI Mendeleev predicted the properties of this element, he called eka-silicon

11. Eka-Silicon PropertyPrediction AppearanceDark-grey Relative Atomic mass 72 Density5.5 g cm -3 Reaction with waterNone Reaction with alkaliMore than with acid Reaction with AcidVery little OxideBasic ChlorideLiquid, <100 o C b.p.

12. Germanium PropertyGermanium 1885 AppearanceDark-grey Relative Atomic mass 73 Density5.35 g cm -3 Reaction with waterNone Reaction with alkaliMore than with acid Reaction with AcidVery little OxideBasic ChlorideLiquid, <100 o C b.p. Mendeleev also predicted the properties of Gallium and Scandium

13. What was the faulty reasoning that led to inaccurate relative atomic masses (atomic weights)? There were two main faults. First chemists were not distinguishing between the weights of atoms and of molecules. Seven common elements exist as diatomic molecules. Of special importance was hydrogen, the original standard for atomic weights. If a molecule of H 2 is given a relative mass of 1 instead of 2, then when other elements are compared with it, their relative atomic masses are halved. Second, at the time chemists used a term called equivalent, or combining weight. This was the number of grams of an element that combined with 8 g of oxygen (easier to do than with hydrogen) (They used this because 8 g of oxygen combine with 1 g hydrogen so 8 g of oxygen was equivalent to 1 g hydrogen.) Relative Atomic Masses