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History of the Periodic Table

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Introduction

´╗┐History of the Periodic Table TEP041 ?Discuss the history and modern rationale behind the arrangement of the elements in the modern periodic table, indicating how the blocks and relative positions relate to the element?s nature, electron configuration, valency and their general chemical properties.?? In ancient Greece, the philosopher Aristotle put forward the idea that there were four main elements; earth, fire, air and water. This theory stood strong until the early 19th Century when true chemical elements started being discovered and it was realised that a way of categorising the elements according to their physical and chemical properties was needed. These groupings were to become the periodic table. In 1829, Johan Dobereiner classified the elements into groups of three, or as he called them, triads. The elements which were grouped in each triad exhibited similar chemical properties and orderly physical properties. For example, chlorine (Cl), bromine (Br) and iodine (I) were a ?triad? as were calcium (Ca), strontium (Sr) and barium (Ba). Dobereiner proposed the Law of the Triads which states that: ?when elements are placed in order of the ascending order of atomic masses, groups of three elements having similar properties are obtained. The atomic mass of the middle element of the triad is equal to the mean of the atomic masses of the other ...read more.

Middle

Because of this Mendeleev?s table was generally accepted and he became known as the ?Father of modern chemistry?. Mendeleev?s periodic table was very successful, although did show to have a few problems; Firstly, the positions of isotopes could not be placed in the table. As an example, normal carbon or 12C would fit perfectly in the table. An isotope of carbon, 14C however, exhibits the same characteristics as 12C would not fit in the table as it would have to be accommodated along with nitrogen, in the same position. Secondly, in order for the elements to fit the requirements of the Mendeleevillian table, those in a particular column have the same valance. Mendeleev in certain cases put an element which had a higher atomic weight ahead of one which had a slightly lower atomic weight. To keep tellurium (Te) in the valance 2 column and iodine (I) in the valance 1 column, tellurium was put ahead of iodine, even though it had a higher atomic weight. This was to later make part of the modern periodic table. The successes of the table were that although when Mendeleev presented his table, the Nobel gases were still undiscovered, as they were found, they could be put neatly as the last group of elements without disturbing the rest of the table. ...read more.

Conclusion

Groups 1A to VIIIA hold the main group elements while 1B to VIIIB holds the transition metals. The lanthanide and actinide series are known as the inner-transition elements. The modern periodic table is roughly split into metals and non-metals. Those which exhibit the most metallic properties such as the alkali metals are placed on the far left of the table while the non-metals occupy the right of the table. The Nobel gases which are completely inert due to having full outer electron shells are set at the far right of the table in group 18. The transition elements, which are a bridge between highly metallic alkali elements and the non-metals, lie in the centre of the table and exhibit characteristics of both metals and non-metals. As the elements move from left to right across a period, the metallic character or the tendency to lose electrons decreases but as they move down a group, this tends to increase. This is due to the fact that there is a general increase in the ionisation energy moving across a period whereas there is a decrease moving down the group. As we move from left to right, we see the reverse and the elements tend to become more metallic, losing their non-metallic properties. Similarly, as they go down a group, non metallic character tends to decrease. ...read more.

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