Periodic Table http://www.markbardsleyillustration.co.uk/

Lavoisier 1789 Traite Elementaire de Chimie. http://web.bilkent.edu.tr/ Produced the first table of elements Introduced a logical system for naming compounds and helped introduce the metric system May 8th 1794

Atomic Weights 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. Hydrogen 1 Carbone 4.2 Oxygen 5.5 Water 6.5 Sulphur 14.4 Sulphuric Acid 25.4 www.bioanalytical.com

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

Dobëreiner 1817 Noticed that certain groups of 3 elements, www.eoearth.org Noticed that certain groups of 3 elements, e.g. Ca, Sr, Ba the atomic weight of 2nd was approximately the mean the 1st and 3rd. Dobëreiner’s triads (5 in total) Element Relative atomic mass Lithium 7 Sodium 23 Potassium 39 Element Relative atomic mass Calcium 40 Strontium 88 Barium 137

Béguyer de Chancourtois 1862 Telluric Screw fr.wikipedia.org 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.

Newlands www.chemsoc.org Newlands in 1865, using Cannizzaro’s system (elements in order of succession) of atomic weights noticed a pattern, noticed that the 8th one was a ‘kind of repetition of the 1st. He called this the ‘Law of Octaves’. OK for the first 15 or so elements Element Atomic weights Atomic Weights Hydrogen 1 Fluorine 8 Chlorine 15 Lithium 2 Sodium 9 Potassium 16 Beryllium 3 Magnesium 10 Calcium 17 Boron 4 Aluminium 11 Chromium 18 Carbon 5 Silicon 12 Titanium 19 Nitrogen 6 Phosphorus 13 Manganese 20 Oxygen 7 Sulphur 14 Iron 21

Meyer www.apsidium.com 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. Element I Atomic weights II III Atomic Weights B 11 Al 27.3 C 11.97 Si 28 N 14.01 P 30.9 O 15.96 S 31.98 F 19.1 Cl 35.38 Li 7.01 Na 22.99 K 39.04 ?Be 9.3 Mg 23.9 Ca 39.9

Mendeleev 1871 Unlike Meyer, Mendeleev 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. elmoscow.ru

Mendeleev 1871 period I II III IV V VI VII 1 H 2 Li Be B C N O F 3 Na K Mg Ca Al * Si Ti P S Cr Cl Mn Fe Co Ni 4 Cu Rb Zn Sr Y Zr As Nb Se Mo Br Ru Rh Pd 5 Ag Cd In Sn Sb Te Mendeleev predicted the properties of this element, he called eka-silicon

Eka-Silicon Property Prediction Appearance Dark-grey Relative Atomic mass 72 Density 5.5 g cm-3 Reaction with water None Reaction with alkali More than with acid Reaction with Acid Very little Oxide Basic Chloride Liquid, <100oC b.p.

Germanium Property Germanium 1885 Appearance Dark-grey Relative Atomic mass 73 Density 5.35 g cm-3 Reaction with water None Reaction with alkali More than with acid Reaction with Acid Very little Oxide Basic Chloride Liquid, <100oC b.p. Mendeleev also predicted the properties of Gallium and Scandium

Relative Atomic Masses 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 molecules. Seven common elements exist as diatomic molecules. Of special importance was hydrogen, the original standard for atomic weights. If a molecule of H2 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 (easierto 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.)