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The Development of the Periodic Table

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In the early 19th century many chemists began to develop their knowledge of analytical chemistry, the classification of compounds, and it soon came necessary to classify the elements. Johan D�bereiner showed in 1817 that atomic weight of strontium lies approximately between that of calcium and barium and that these element showed a number of similar properties and thus should be grouped. He and other leading chemists later went on to show how this was also true for the halogens and the alkali metals.

In 1863 John Newlands showed that when the elements were arranged in order of atomic weight every eighth element showed familiar traits and thus a table idea was brought up. He considered that some elements had not been invented so he left gaps to fit his idea however after about 20 elements the table became inaccurate.

In 1869 Dimitri Mendeleyev did extensive research to traits of elements, especially valency, and developed his own table which left gaps for undiscovered elements. He also changed the order of some elements as their properties fitted better that way which lead to doubt in the accuracy of the atomic mass of elements and if element should be sorted by mass. For these undiscovered he predicted what properties they would have.

When gallium was discovered by Paul Emile Lecoq de Boisbaudran in 1875 Mendeleyev’s idea of periodic law was excepted as the properties of gallium matched those that Mendeleyev predicted.

Ernest Rutherford’s work on nuclear charge lead to the proof of the existence of sub-atomic particles which lead to great advances in the understanding of elements. Today’s table gives each element an atomic number based on the number of protons which the elements are ordered on. Neils Bohr’s work on atomic structure showed that different periods are the result of the energy shells that the electrons are in when in they’re in their ground state. However Mendeleyev is still credited as the inventor of the periodic table. Here is a modern version of the periodic table showing where gallium was discovered and how the atomic mass of iodine is less than that of tellurium.

Figure 1 Modern version of the Periodic Table of Elements (G. P. Moss (2003), www.chem.qmul.ac.uk/iupac/AtWt/table.html)

Gallium is a silvery, glass-like, soft metal with some very unusual properties that make it unsure weather or not gallium is a metal or not. Firstly its physical properties are unusual as it has a low melting point, unlike most metals of just a few degrees above room temperature. Like most metals it has high boiling point of 2676K giving gallium the widest liquid range then any other element. Gallium will also alloy with most metals to create low-melting alloys and good liquid alloys to use in thermometers. More recently gallium has been used in doping semiconductors, producing solid-state devices such as transistors and gallium arsenide is a key component of LEDs. Unusually for a metal, gallium is denser as a liquid then as a solid, like water.

The chemical properties of gallium are much the same of aluminium, above it in the periodic table, as Mendeleyev predicted. They both dissolve in both acids and alkalis which is unusual for a metal. The reactions of gallium and aluminium with an acid and an alkali are shown below showing that aluminium and gallium behave in identical ways.

Acid (H+ ions)

2Al(s) + 6H+(aq) –> 2Al3+(aq) + 3H2(g)

2Ga(s) + 6H+(aq) –> 2Ga3+(aq) + 3H2(g)

Alkali (OH- ions)

2Al(s) + 2OH-(aq) + 6H2O(l) –> 2[Al(OH)4]-(aq) + 3H2(g)

2Ga(s) + 2OH-(aq) + 6H2O(l) –> 2[Ga(OH)4]-(aq) + 3H2(g)

After the discovery of gallium many scientists went to work at discovering new elements to fill in gaps in Mendeleyev table however a greater understanding of the elements was required and an understanding of the structure of atoms. Atomic spectroscopy and the UNILAC accelerator have been used to increase this knowledge of element so that scientists can develop new elements.

It wasn’t until just before the turn of the 20th century that Ernest Rutherford proved that sub-atomic particles existed which lead to great advances in the understanding of elements.

Neils Bohr’s work in 1913 on atomic emission spectroscopy looked at the emissions of light when electrons from excited states fall back to a lower state releasing a quantum of energy in the form of light based on the quantum theory. Different transitions between energy levels results in different frequencies of light which is different for every type of element hence characterising the element. As visible light is only released when electrons fall back to the 1st excited level (the Lyman series) only emissions from this series are observed.

De Boisbaudran discovered gallium before quantum theory however emission spectra were thought to be different for different elements. He noticed a frequency of light in the spectrum of zinc sulphide ore did not match any known element, this was gallium. He managed to obtain one gram of nearly pure gallium which enabled him to determine some of gallium’s properties. This is the emission spectrum of gallium showing the line de Boisbaudran found in the zinc sulphide ore. He later went on to find two other elements in this way.

Figure 2 Emission spectrum of Gallium (Mikolaj Pytel (2001), Spektrus 1.0)

Although atomic spectroscopy found more elements there are still elements to be discovered which cannot be found using this method as they do not occur naturally as they are not at all stable. It has now become a challenge for scientists to synthesise new elements.

When a large element is bombarded by a smaller element travelling at a very fast speed the two nuclei may fuse together to form a new element with the combined amount of protons. Here is a diagram and the equation of two nuclei fusing together:

This method has found many of the new elements since 1965. However as the elements get bigger the nuclei needed greater violence between them to persuade them to react. The result of this process is a radioactive element that usually decays quickly and so most of the elements are useless however due to trends in known elements there are certain elements that are very stable that appear in the table at sensible points. Scientists believe that ununquadium-298 (114 protons) will be stable and will be very valuable.

To create heavier elements requires accelerating the nuclei very fast which is currently done by GSI with a universal linear accelerator (UNILAC) into a rotating disc of target metal and the decay can then be analysed to determine what element has been produced.

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