Chemistry Essays & Paper Examples

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Essay On Mendeleev & The Periodic Table

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The periodic table was developed by Dmitry Mendeleev in 1869. It classifies the elements according to their properties and increasing atomic weight. The atomic number, which identifies an element s position in the periodic table, is equal to the amount of protons in the nucleus. From spaces in his table, Mendeleev was able to predict the existence of elements not yet discovered. Dmitry Mendeleev was a Russian chemist, best known for his development of the periodic law of the properties of the chemical elements. This law states that elements show a periodicity of properties when they are arranged according to atomic weight. Mendeleev was born in Siberia. He studied chemistry at the University of Saint Petersburg, and in 1859 he was sent to study at the University of Heidelberg. There he met the Italian chemist Stanislao Cannizzaro, whose views on atomic weight influenced his thinking. Mendeleev was a renowned teacher, and, he wrote the book Principles of Chemistry, which became a classic. During the writing of this book, Mendeleev tried to classify the elements according to their chemical properties. In 1869 he published his first version of what became known as the periodic table, in which he demonstrated the periodic law. In 1871 he published an improved version of the periodic table, in which he left gaps for elements that were not yet known. His chart and theories gained increased acceptance when three predicted elements gallium, germanium, and scandium were subsequently discovered. English scientist John Dalton realised that what distinguished an atom of one element from an atom of another was its weight, when he put forward his atomic theory. Soon after Dalton published his atomic theory, Italian chemist Amadeo Avogadro showed that it made sense to suppose that a fixed volume of gas, whatever its chemical constitution, always contained the same number of particles atoms or molecules. This was Avogadro s hypothesis . It was largely ignored at the time, but it became very important 50 years later. Swedish chemist J ns Jacob Berzelius was the first scientist to make a systematic investigation of relative atomic weights. He published a list of weights, based on two standards; by giving hydrogen a weight of 1, and oxygen a weight of 100. There were some errors in the list. Berzelius identified some compounds as elements, but the idea that the atoms of elements differed from one another in atomic weight took a firm hold among chemists. Mendeleev and perhaps other chemists had a monumental idea to use the atomic weights of the elements to construct some organising principle that could relate their properties to their atomic weights. Mendeleev went to work trying to discern a pattern among the elements. When he arranged the elements according to their atomic weight, he found a pattern not discovered by his predecessors. The pattern had to do with the valence, or combining power, of the elements. He discovered a periodic rise and fall of valences, and this led him to construct a table of the elements. The table contained rows, one on top of the other, which reflected the rise and fall of the valences. The columns this produced possessed elements that had similar chemical properties. For example, elements in the first column included lithium, sodium, and potassium, all of which had a valence of 1 and all of which had similar chemical properties. Among the next column s elements were beryllium, magnesium, and calcium, all with a valence of 2 and chemically similar. Mendeleev called the members of each column a family, to imply a kinship. Today, each column is called a group. Mendeleev gave priority to similarity of properties over atomic weight, and he assigned positions for some elements in columns according to their properties, even though their atomic weights seemed to place them out of order. He also suggested that the determination of the masses of these elements might be an error. They turned out to be correct, or near enough correct, so that in these cases elements with greater weights preceded elements with lesser weights. However, this did not counteract the general validity of Mendeleev s table. There was an explanation for these exceptions, but this was not found out until 1913 when English physicist Henry Moseley investigated the X-ray spectra of various atoms. Moseley found that the wavelength of the X rays emitted related directly to the atoms position in the periodic table. This was later found to be due to the protons in the nucleus, which is the atomic number of the element. The discrepancies in atomic weights were proven later to the presence of naturally occurring isotopes (atoms with the same atomic number but different atomic weight). When Mendeleev finally refined his table so that it included all the then known elements, something perplexing appeared. There were empty places between certain elements. He had deliberately left the places blank, because there were no elements whose properties justified their being in those places. Critics leaped on these omissions, claiming they proved that the concept was seriously flawed. Mendeleev countered by stating that the missing elements would be discovered. Moreover, he boldly predicted what some of their chemical and physical properties would be. Mendeleev had predicted the properties of three elements; Germanium, Gallium, and Scandium. When these elements were discovered, they were found to possess the properties he had predicted for them. The weight of this evidence was overwhelming, and the periodic table was finally accepted as the organising principle of the elements. Certain changes have since been made to the periodic table. For example, with the discovery of the electron, and the configurations of the electrons around the atoms of different elements, it became apparent that the arrangement of electrons varied periodically throughout the table. The modern periodic table lists the elements in order of their increasing atomic number, but it organises them into periods and groups according to their electron configurations.

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