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History of Periodic Table

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Chemistry (Salters) Open-Book Paper 14 February 2004 History of Periodic Table German chemist Johann Dobereiner was the first to attempt to classify elements using their atomic weights (now known as relative atomic mass). He identified triads in the list of elements, for each triad the atomic weight of the middle element was approximately the average of the other two. In 1863, John Newlands noted that when the elements were written in order of increasing atomic weight, every eighth element had similar properties. So Newlands tried to produce a new periodic table based on his discovery but after about 20 elements his table became ragged; he had to put two elements in one space and some elements were in the wrong place because Newlands used inaccurately measured atomic weights. Still, Newlands was quite close to the right answer. ...read more.


He named these undiscovered elements and predicted their properties in immense detail. In 1875 a French scientist, Paul Emile Lecoq de Boisbaudran, discovered a new element that he named gallium. One of the elements that Mendeleev predicted was named eka-aluminium (as it was placed below aluminium in Mendeleev's periodic table) and amazingly this new element gallium shared near identical properties to Mendeleev's eka-aluminium. Mendeleev's idea about the periodic table was given an enormous boost by this discovery. Over the next ten years two more of Mendeleev's predicted elements were discovered; scandium (predicted by Mendeleev as eka-boron) and germanium (predicted as eka-silicon). Unusual Properties of Gallium: Gallium has three intriguing physical properties. Firstly, gallium's melting point is 29.78�C, which is only slightly above UK room temperature. ...read more.


Figure 2 - Reaction of gallium with hydroxide ions Gallium is one of very few metals which have amphoteric hydroxides (hydroxides that show both basic and acidic properties because they react with both acids and alkalis). However gallium does also have properties that resemble non-metals. For instance, Gallium has a structure extremely similar to the non-metal iodine. Iodine and gallium both have a similar orthorhombic structure. Gallium is also a semi-conductor. From Discovering Elements to Synthesising Elements The UNILAC accelerator The UNILAC accelerator works by firing a beam of relatively heavy ions from an accelerator towards a rotating disc of the target (a heavy, stable element such as lead). If done with sufficient violence, the ions overcome the natural repulsion of nuclei and fuse together, generating a new element. The new elements are then separated and detected. ...read more.

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