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The history of the periodic table

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In the early 19th Century, Johann D�bereiner made the first attempts to classify elements using their atomic weights and identified a number of 'triads'. In 1863, John Newlands discovered the Law of Octaves, where by he realised that when the elements were written in order of increasing weights every 8th element had similar properties. However, he had no knowledge of the noble gases and so therefore there were only seven elements in each period. Also, some of the atomic weights had been measured inaccurately and so he had difficulties placing elements beyond the first 20. In 1869, Dimitri Mendeleev revised some atomic weight values and left gaps for undiscovered elements to produce a more accurate version of the modern periodic table. He predicted properties for five of these undiscovered elements and three of these predictions were very accurate. Mendeleev's ideas of the periodic table were supported by the discovery of gallium in 1875 by Boisbaudran. He used atomic spectroscopy to examine a zinc sulphide ore, which also contained one of the new elements, gallium. Gallium was one of the elements which had been predicted by Mendeleev, who had named it eka-aluminium. Boisbaudran measured the properties of gallium and found that they were very similar to the predicted properties of eka-aluminium. This was supporting evidence for the correctness of Mendeleev's periodic table. (Ref. Page 5 of open-book exam paper) (Word-count 228/994) Since Mendeleev's version of the periodic table, the modern periodic table has been developed based on his work, where the gaps he left for undiscovered elements have been discovered and filled in. A new group, the noble gases has also been introduced. The atoms are now arranged in order of increasing atomic number rather than increasing atomic mass. Also, an understanding of electronic configuration has helped to explain similar properties in vertical groups, transition metals and trends across the periods. Gallium is considered to be an unusual element because of its properties. ...read more.


It has a very low melting point of 29.78�C, so that in hot countries it becomes a liquid metal, like Mercury, at room temperature. It has a very high boiling point of 2,403�C, and because of its low melting point, it has the widest liquid range of any element. This means that it can be used in high-temperature thermometers because it remains in a liquid state over a wide range of temperatures. Like water, it is denser as a liquid than as a solid at its freezing point. This also means like water, the solid form has a more open structure than the liquid and so it solidifies from the top down. Table 1 (shown above) summarizes the main chemical properties of gallium. Gallium is classified in the 'other metals' group of the periodic table. However, it has some non-metal properties as well as metal properties. "A metal is now defined as an element which can ionize by electron loss" (Ref. A New Certificate Chemistry, p.469 - Holderness and Lambert). This is modelled by the following reaction: Metal atoms + Hydrogen ions � Metal ions + Hydrogen molecules M(s) + 2H+ (aq) � M�+ (aq) + H2 (g) The following reaction involving gallium, therefore suggests that it is a metal 2Ga(s) + 6H+ (aq) � 2Ga�+ (aq) + 3H2 (g) However, a chemical characteristic of a non-metal is that they form covalent chlorides. This is typical of gallium as it forms its tri-chloride: Ga2Cl6 (Ref. Page 6 of open- book exam paper) This property of gallium suggests that it is a non-metal. (Word-count 353/994) Atomic spectroscopy is an in-depth measurement method relying on the spectroscopic process of excitation and emission. Atoms are capable of undergoing electronic transitions due to absorption of light or through excitation and emission of light. Since atoms have no rotational or vibrational energy, these transitions are a function of the quantized energy levels available for the atom. ...read more.


(Ref. Insert 2 of exam paper) The UNILAC accelerator fires beams of fast heavy ion into metal targets with great force in an attempt to encourage the nuclei to fuse together to form new elements. Both of the techniques described above have been used to increase our knowledge about chemical elements in the periodic table as atomic spectroscopy has helped to discover new elements, where as the UNILAC accelerator has helped to synthesise 'artificial' elements. However, both techniques also rely on an understanding of the structure of atoms as they are both based on the idea of electronic configurations, where the electrons are arranged into sub-shells, which have different energy levels. When an atom is excited, electrons jump into higher energy levels and then later they drop back into lower levels and emit the extra energy as electromagnetic radiation, which gives an emission spectrum like the one in Fig 4 (shown above). This is based on Neils Bohr's theory which explained how we get absorption and emission spectra and gave scientists a model for the electronic structure of atoms. Bohr's theory was based on the idea of quantisation of energy and supported the quantum theory, which describes the movements and energies of particles on an atomic scale. Initially, when Mendeleev left gaps in the periodic table for 'missing' elements scientists were concerned about discovering these elements and so filing in the gaps to complete the table. However, as their knowledge grew about the arrangement of electrons in elements, it helped to explain the patterns of behaviour and properties of the elements. (Word-count 336/994) Ideas about electronic configurations led scientists to being able to predict the properties of elements with certain atomic numbers, accurately, and therefore wanting to discover these elements. This meant after uranium, the last naturally occurring element, scientists had to synthesise 'artificial' elements, to increase the number of elements in the periodic table. An example of scientists first predicting the properties of these artificial elements and then synthesising them is element 114, which is still being worked on. ...read more.

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