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Salters Open Book Paper April/May 2004 One of the earliest attempts to organise chemical elements was that of Johann Dobereiner, who was a German scientist. He noticed that there were several groups of three elements that had similar properties, and that the middle of these (when ordered by atomic weight) had a weight roughly equal to the mean of the other two. Although Dobereiner's contemporaries looked upon these discoveries as mere coincidences, they were important and the idea of trends in the properties of elements was born. Triad 1 Triad 2 Name Atomic Mass Name Atomic Mass First element calcium 40.1 lithium 6.9 Third element barium 137.3 potassium 39.1 Average mass 88.7 Average mass 23.0 Second element strontium 87.6 sodium 23.0 Doberiener's Law of Triads [1]. Note the similarities between the average mass of the first & third elements, and the second. All the elements in a given Triad have similar chemical and/or physical properties. In 1863, the British scientist John Newlands noticed that a pattern occurred in every eighth element when they were organised in order of increasing atomic mass, note that at this point the noble gases were yet to be discovered. ...read more.


The modern Periodic Table is the result was a combination of many scientists' work. Although it was invented by Mendeleev, it followed from ideas first proposed by Newlands and Dobereiner, and was revised by Mosely, who realised that it worked much better if the elements were arranged by atomic number as opposed to atomic weight. The French scientist Paul Emile Lecoq de Boisbaudran discovered gallium in 1875, by a technique known as atomic emission spectroscopy. This uses an electric arc to excite atoms. The excited atoms then emit light and a prism is used to split this so that the emission spectrum for the sample is shown. The atoms absorb photons of radiation and use this to promote electrons to higher energy levels. The energy level can be described as quantised, that is; fixed quantities of energy are absorbed or emitted when electrons move between different energy levels. As each element has a different electronic structure, each element therefore also has its own unique emission and absorption spectra. When de Boisbaudran found a new, unique, set of emission spectra it was obvious that he had discovered a new element. ...read more.


Increasingly, more and more developments in the world of chemistry are adding to our bank of knowledge concerning atoms, their structure and the ways they behave. It is necessary to have a comprehensive knowledge of these things, in conjunction with a great deal of ingenuity and luck in order to be able to make new and exciting discoveries. Using the (incorrect) theory that atoms are the smallest particles of matter, such techniques, as emission spectroscopy would never have been developed, as this requires complex knowledge of sub-atomic particles and their behaviour when energised. Quantum theory is very important in the development of the Periodic Table as it explains the structure of the atom and the movements and energies of particles on an atomic scale, which is very different to the rules of physics governing the macro world we experience on a day to day basis. Modern chemists have moved from merely discovering elements to creating them. In order to do this; the UNILAC accelerator is required. A beam of heavy ions are fired at a sample of a heavy yet stable element, lead is a common choice. ...read more.

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