One of these elements was eka-aluminium, and as suggested by his name was predicted by Mendeleev to be below aluminium in his periodic table. On its discovery, twelve years later by the French scientist, Lecoq Boisbaudran, who named it ‘gallium’, the remarkable similarities observed between Mendeleev’s predictions and its actual properties confirmed Mendeleev’s ideas of the periodic table as being correct.
Comparing Mendeleev’s predictions with the properties of element 31, gallium (Ga)
(Table and title taken from Salter’s open book paper2004, article1, page 5, table 1)
Gallium has very unusual properties both physical and chemical. It has an exceptionally low melting point for a metal, and its liquid state at such a low temperature is shared by only a few other metals. It possesses the property of having the largest liquid range, with a boiling point of 2403°, which is very strange for a metal, as metals are considered to be solids rather than liquid in the first place. It is probably the fact that solid metal is less dense than liquid form, which is the most bizarre property of gallium. This unique property, which gallium shares with water, suggests that gallium is a non metal rather than a metal, which all are denser as a solid than a liquid.
Image from www.vanderkrogt.net/elements/ elem/ga.html
Gallium’s chemical properties are closely similar to those of aluminium. Like aluminium, it reacts with both acids and alkalis to give hydrogen.
The reaction shows that gallium produces amphoteric hydroxides, which is something very few metals do. Another chemical property of gallium which is rare amongst metals, is the formation of anhydrous trichloride, (Ga2Cl6), one that is also shared with aluminium. Both these properties suggest that gallium is not a metal. Another strange property which also shows this, is the strange structure of gallium chloride (empirical formulae GaCl2), which can be written as GaCl.GaCl3.
It is atomic spectroscopy that enabled the discovery of gallium and many other elements. The method discovered in 1860, involves the absorption and the emission of electromagnetic radiation by electrons and their transmission between electron shells. This is done using an atomic emission spectrometer, in which an electric arc excites the electrons of free atoms, which move up energy shells. On moving to a lower state, each electron emits a photon, and those with frequencies within the visible region, can be viewed through a prism as a series of straight coloured lines. Each element within the periodic table has its set of lines, or emission spectra, which can be used to identify its presence within a substance. It was with the use of the understanding of atomic structure, which enabled scientists to use atomic spectroscopy in order to discover new elements. This made it possible for the spaces left by Mendeleev to be filled, and with each discovery the knowledge on chemical elements extended.
By the Second World War, all naturally occurring elements had been discovered, and scientists began to idealise synthesising new elements to add to the periodic table. This brought about the invention of the UNILAC accelerator, which allowed scientists to fuse atomic nuclei together to produce nuclei of new elements. It is evident that the understanding of atomic structure was used by the inventors, as the apparatus relies entirely on the theory that overcoming the natural repulsion between two nuclei will allow them to fuse together as one. This is done by firing a heavy ion beam at another heavy element such as lead. If the beam bombards the heavy element at a high enough force, the nuclei will fuse together, to produce a new element.
The UNILAC accelerator and the method of fusing atomic nuclei have been used to synthesise elements 92 to 114 of the periodic table over the last 25 years. This has not only broadened our understanding of chemical elements, but also allowed us to view atomic structure from a different perspective.
After the discovery of gallium in 1875, atomic spectroscopy enabled the discovery of the remaining naturally occurring elements and was used until the early 20th centuries by scientists as the primary method of finding new elements. However, after this no more additions could be made to the periodic table using atomic emission spectrometry, and thus scientists changed their concept of discovering elements to synthesising new elements. This brought about the invention of the UNILAC accelerator, which allowed them to fuse two existing atomic nuclei into a synthesised nucleus of a new element.
Bibliography
Salter’s open book paper 2004, article 1 and article 2 pages 3-9
Salter’s Chemical storylines textbook, EL3 - Looking for patterns in elements, pages 7-9 (excluding EL4 text page 9)
Salter’s Chemical ideas textbook, CI 6.1- Light and electrons, page 124-125 (subheading - atomic spectra)
http://www.webelements.com/webelements/elements/text/Ga/key.html
http://www.upei.ca/~physics/p221/pro00/periodicTble/page2.html
http://web.fccj.org/~ethall/period/period.htm (Image 1 - Newlands’ table of elements taken from this website)
http://www.chemsoc.org/viselements/pages/data/gallium_data.html
