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History of Chemistry and Atomic Structure.

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Introduction

History of Chemistry and Atomic Structure Greeks - First to consider the nature of matter (400 BC) 1) Two camps: a) matter is continuous - keep on dividing matter indefinitely; you just get smaller pieces Aristotle believed that matter was continuous b) matter is finite; it is composed of small indivisible particles Democritus - everything made up of atoms (atoms - Greek for "indivisible") 2) 4 different types of "elements" or "atoms" : Fire, Earth, Water, Air How were these 4 types of "atoms" different - they were especially interested in shape since Greeks were really into geometry: Fire - jagged shaped, since fire hurt Water - spherical, smooth since water flowed easily Earth - cubical - earth was solid and stable 3) The Greeks were not doing science as we know today but an intellectual exercise (philosophy) since no evidence existed one way or the other and they had no experiments to test their ideas (science) 4) Aristotle held more prestige and his ideas about matter held on for almost 2000 years. ...read more.

Middle

and the ash that was left could not (all the phlogiston was gone). Stahl saw a similarity with rusting iron which he said was simply a slow burn. The iron lost its phlogiston as it rusted and could rust no more when the metal was gone since it had lost all its phlogiston. Things looked promising since wood lost weight upon burning. Of course it should, since it was losing phlogiston. But, iron GAINED weight upon combustion (rusting). If lost phlogiston, how could it weigh more? It quickly became obvious that exact measurement in chemistry was essential to understanding what was going on during chemical reactions. Quantitative analysis (Late 1700s) In the 1790's, accurate measurement of masses became widespread for the first time due to improvement of balances. This led scientists to measure the masses of the elements and compounds they were working with. What followed was the establishment of several new laws of chemistry: 1) Law of conservation of mass : "In a chemical reaction, mass is neither created nor destroyed." Antoine Lavoisier of France carefully weighed the reactants and products of various chemical reactions and formulated this law. ...read more.

Conclusion

atoms must contain other particles since electrons weigh so little Discovery of the atomic nucleus - Ernest Rutherford (1911) 1) Thin gold sheets were bombarded by alpha particles ( radioactive decay particles - 2 protons, 2 neutrons and 4 times the mass of a hydrogen atom and positively charged) 2) Mass and charge were assumed to be uniformly distributed in the atom, so alpha particles were expected to pass through with slight deflections. Unexpectedly, there were some large deflections and 1 in 8000 actually bounced straight back! See pg 72 of your text 3) Result - a) the atom had a center of concentrated positive charge b) the atom was mostly empty space - if nucleus was the size of a BB, size of atom was the size of a football stadium c) this would imply that electrons orbit the nucleus like planets around the sun in this "empty space" (found later to be wrong) Atomic Structure - a summary 1) The nucleus contains protons with a +1 charge and neutrons with no charge. Both have about the same mass (neutrons are 0.14% heavier). 2) Electrons are somewhere outside the nucleus and have a -1 charge. It takes about 1836 electrons to equal the mass of 1 proton. ...read more.

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