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The Development of the Periodic Table.

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Chemistry Open Book Exam: The Development of the Periodic Table By Osman Khan Lee 12 Early attempts at the Periodic Table Johann Dobereiner The first scientist to classify the elements into similar properties was the German chemist, Johann Dobereiner. He attempted to arrange them in terms of atomic weights- which we now call relative atomic mass. He identified a number of triads which he classified as having the same properties. His discovery enabled him to identify that the mass of the middle element was approximately the average of the other two. E.G. Element Relative Atomic Mass Lithium 6.9 Sodium 23 Potassium 39.1 The table above shows a triad. The middle element's relative atomic mass can be determined by calculating the average of the two around it: RAM of Sodium = (6.9 + 39.1) / 2 = 23 As well as this there were several other triads identified. However, people believed that this theory was merely coincidence, and was therefore not taken very seriously. Dimitri Mendeleev A few years later Dimitri Mendeleev enhanced Newlands periodic table, which was arranged in order of atomic mass but had several flaws. He corrected the weight values of some of the elements which Newlands had incorrectly calculated but also he realised some elements had not yet been discovered. For this reason he left gaps, which Newlands did not. ...read more.


As a result only a small amount of energy is needed to be put in to make it melt. On the other hand gallium has a boiling point of 2403*c, giving it the largest liquid phase range of an element. This may be due to the fact that gallium's intermolecular forces give it added strength, so more energy must be put in to break these bonds. If gallium is a diatomic element, then the instantaneous dipole-induced dipoles that form between molecules means more energy is required to give the molecules sufficient energy to overcome one another's attractive forces. Below is diagram showing this. 4Diagram showing how a molecule can become polarised and form dipoles Instantaneous dipole- induced dipole will form here and will hold the two atoms together more strongly. This will mean more energy is needed to be put in to separate the two atoms. The same idea will occur for Ga2. Electrons, being negative, shall repel one which will cause the electron density in both atoms to move to opposite sides. As a result one part of each atom shall become positive and the other negative. Metal or Non-Metal/ Chemical Properties Gallium also has some interesting chemical properties. It dissolves in both acids and alkalis, which is very rare. It therefore shows it has both basic and acidic properties. This is also true for aluminium which lies in the same group as gallium. The equations below show that gallium (and aluminium) ...read more.


We can therefore explain trends found in the periodic table. Two elements which have the same number of electrons in their outer shells will react in a similar way, as they both need to gain/lose the same number of electrons. Changing Work of Alchemists Over the last two hundred years there have been significant changes in what alchemists now do. The nineteenth century saw the main focus on discovering elements and thinking how to classify elements. Mendeleev was particularly important for the classification of the known elements, whilst others, such as Paul Emile Lecoq who discovered gallium, worked to complete the periodic table which was being established. Today the main focus of alchemists is to synthesis new elements. This is done by firing beams of metal ions into metal targets in order to encourage the nuclei to fuse, and thus creating a new element. This is done using a UNILAC Accelerator. 8Diagram showing the stages of gold and nickel nuclei fusing together to form bohrium Gold nuclei (79 blue protons) Nickel nuclei (28 red protons) Nickel ions are fired onto a rotating disc of the target metal (gold in this case). Nickel ions are fired with enough speed to overcome the repulsion of the nuclei The two elements have now fused together to form bohrium In these ways the UNILAC accelerator has been very useful in synthesising new elements, and shows us how alchemists are now using advanced techniques to discover new elements as opposed to two hundred years ago when much was done by simpler reactions- e.g. displacement and (later) electrolysis. ...read more.

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