1.1 Atomic Structure


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1 UNIVERSITY OF ALBERTA INTRODUCTORY UNIVERSITY CHEMISTRY I (CHEM 101) Atomic Structure Historical development Democritus (460 – 370 BC) first proposed matter was made up of indivisible particles – atoms (Greek atomos , which means “uncuttable”) Late 18 th century: Law of Conservation of Mass – matter is neither created nor destroyed. Law of Constant Composition – A given sample of compound always contains the same fraction of the element by mass. Law of Multiple Proportions – When two elements A and B form more than one compound, the masses of element A that combine with a fixed mass of B are in simple ratio. Dalton put forward his Atomic Theory in the early 19 th century. But the structure of an atom was still not known. Early Experiments to Characterize the Atom The electron Fig 2-4 p 49 By using a Cathode ray tube J.J. Thomson found that: Cathode ray can be deflected by electric and magnetic field - they are negatively charged . Cathode ray is composed of negatively charge particles which possess mass . The mass / charge ratio was found to be -5.6857 x 10 -9 gram / coulomb The particle is called electron and is a fundamental particle of all atoms Further work by Millikan determined the electronic charge = -1.6022 x 10 -19 C
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2 -5.6857 x 10 -9 gram Mass of electron = x -1.6022 x 10 -19 Coulomb = 9.1096 x 10 -28 g Coulomb Thomson proposed his “plum pudding” model of an atom. The negative charge of an atom is neutralized by a diffused cloud of positive charge.
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