https://www.primeeducation.com.au/prime-hsc-tuition/year-11-tuition/year-11-chemistry/ https://www.iitutor.com/courses/preliminary-chemistry/ The development of the Periodic Table as a useful classification of elements paralleled the development of the atomic theory. As our understanding of the atom increased, so did our understanding of the relationship between elements. This will be shown as we study the evolution of the Periodic Table into its modern form. Classifying elements as metals and non-metals In the period 1772-1785, the French chemist Antoine Lavoisier classified the known elements into metals and non-metals based on the physical and chemical properties. Classifying elements into triads In 1829 the German chemist Johann Dobereiner classified groups of three elements into triads based on their similar physical and chemical properties. Example of triads: • Sulfur, selenium , tellurium • Chlorine, bromine, iodine • Calcium, strontium, barium The Law of Octaves In 1864-1865 the English chemist John Newlands published a table of elements arranged in eight columns and seven rows. The elements were arranged in order of the increasing atomic weight. Early versions of the table left spaces for undiscovered elements while other version did not. Newlands established families of elements based on his Law of Octaves, which stated that every eighth element starting from a given one has similar chemical and physical properties. Thus starting with sodium (the 9th element in his table), the next-eighth element (potassium, his 16th element) had similar properties to sodium. Eventually his law broke down for heavier elements. Initially the Chemical Society ridiculed his ideas, by 1887 they had realised the ground-breaking importance of his ideas. In 1871 the Russian chemist Dmitri Mendeleev published a table of elements which consisted of eight columns and twelve rows. His elements were arranged in order of increasing atomic weight and spaces were left for undiscovered elements. His arrangement placed familiar of related elements in the vertical groups. The periodic repeating pattern of the properties of the elements was expressed by Mendeleev’s Periodic Law: ‘the properties of elements vary periodically with their atomic weights’. In 1869 Dmitri Mendeleev in Russia and Luther Meyer in Germany, working independently of one another, published similar scheme for the classification of the elements. Meyer arranged the elements in order of increasing atomic mass and placed them in groups based on their ‘combining power’ and ‘valency’, the number of bonds an element can form. Meyer also plotted graphs of the physical properties of the elements as a function of atomic mass. These graphs clearly demonstrated the periodic variation in physical properties such as melting point, boiling point, density and atomic volume. Henry Moseley (1887-1915) The irregularities in Mendeleev's table were clarified when the English physicist Henry Moseley proposed the concept of atomic number in 1913. The atomic number is the number of protons in the nucleus of an atom (Unit 4.2). This also equals the number of electrons surrounding the nucleus in a neutral atom. It was eventually recognised that the fundamental factor which determines chemical properties is atomic number rather than atomic mass. On the basis of atomic number tellurium, with atomic number 52, and iodine, with atomic number 53, fell into their correct vertical groups. The modern version of the periodic law is better stated: 'When the elements are listed in order of increasing atomic number, similar chemical properties recur periodically.' The periodic variation of elements' properties was better understood when it was realised that these properties are determined by the electron structure of atoms, in particular the valence or outer energy level electron configuration. From this knowledge it is possible to explain a modern version of the periodic table. Historical development of ideas Items in bold print represent discoveries of particular importance to the classification of elements and the development of the periodic table of the elements.