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Outline the development of the modern Periodic Table, describing how it differs from earlier attempts to classify elements and how the discovery of Gallium supported the Mendeleev's ideas about the Periodic Table.

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1) Outline the development of the modern Periodic Table, describing how it differs from earlier attempts to classify elements and how the discovery of Gallium supported the Mendeleev's ideas about the Periodic Table The periodic table has not always been in this universally known form. The elements were discovered over time. By 1860 about 60 elements were known and a method was needed for organization. Many scientists made significant contributions that eventually enabled Mendeleev to construct his table. The periodic table did not end with Mendeleev but continued to take shape for the next 75 years. Dobereiner was the first scientist who attempted to classify the elements based on their Relative Atomic Mass. He grouped elements based on similarities. Calcium (atomic weight 40.1), strontium (88.7), and barium (137.3) possess similar chemical prepares. Dobereiner noticed the atomic weight of strontium fell midway between the weights of calcium and barium: Ca Sr Ba (40.1 + 137.3) � 2 = 88.7 40.1 88.7 137.3 Dobereiner noticed the same pattern for the alkali metal triad (Li/Na/K) and the halogen triad (Cl/Br/I). ...read more.


discovered by Boisbaudran after studying its properties he found that they coincided with those that Mendeleev had predicted a few years earlier for eka-aluminum, the then undiscovered element lying between aluminium and indium in his periodic table. 2) Describe and explain why the properties of gallium are considered to be unusual, identifying the properties that suggest gallium is a metal and those that suggest it is a non-metal. Gallium melts near room temperature and has one of the largest liquid ranges of a metal. (So it has found use in high temperature thermometers). This is because gallium has a high boiling point and a low melting point, and also has a low vapour pressure even at high temperatures. Gallium is denser as a liquid than as a solid. This is probably because the solid gallium has a more open structure than the liquid. Properties that Suggest Gallium is a Non-Metal The low melting point of gallium is a property that suggests it is a non-metal. The fact that it is denser as a solid than a liquid (like water) ...read more.


Such high speed produces enough force for the ions to overcome the repulsion of the nuclei and fuse together, forming a new element. Atomic Spectroscopy 4) Discuss how the work of Chemists has changed over the last two hundred years from discovering the elements to synthesising elements The time when elements were first being discovered, it was important for the chemists at the time to discover all the elements. When most of the elements had been discovered people like Mendeleev, De Boisbaudran Newlands and others started to classify and categorize the discovered elements based on their behavior. After Mendeleev's 'gaps' had been filled in, chemists discovered the noble gasses. Once all the 92 naturally occurring elements had been found, scientists decided to synthesise new 'artificial' elements. Scientists now can turn an element into another, which is a lot more precious, whereas the medieval chemists only dreamt of turning base metals into gold. However, as these elements are unstable with a half-life so short that any atoms of the of the elements may have been may have been present when the earth formed, but have long since decayed away. Hence are only known to us as the products of UNILAC accelerator (see point 3). ...read more.

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