121lec2 - 1 Chemistry 121 Chapter 2 ATOMS MOLECULES AND...

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Chemistry 121 Chapter 2 ATOMS, MOLECULES AND IONS 2.1 The Atomic Theory of Matter Atoms are the fundamental building blocks of matter. Greek philosophers argued about the question of whether matter can be subdivided into fundamental particles. Democritus (460 – 370 BC) – all matter can be divided into indivisible atomos John Dalton was the first scientist to propose a theory of the composition of matter, based on experimental facts. John Dalton’s Atomic Theory: o Each element is composed of atoms. o All the atoms of a particular element are identical. The atoms of different elements are different. o Compounds are formed when different atoms are chemically bonded. o During chemical reactions atoms are only rearranged, but they are unchanged. Atoms cannot be created or destroyed during chemical reactions. o Atoms are the building blocks of matter. Chemical laws known to Dalton: o The Law of constant composition (or the Law of Definite Proportions): 1 Study material for this chapter: Textbook sections 2.1 through 2.7. These lecture notes must be used in conjunction with the textbook.
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o The composition of a pure compound is always the same. The Law of Conservation of Mass: o The total mass of all the materials before and after a chemical reaction is always equal. o Example: CH 4 + O 2 CO 2 + H 2 O o The conservation of mass means something can neither be created nor destroyed. The Law of Multiple Proportions: o If two elements can form two or more different compounds, the masses of the one element combining with a fixed mass of the other element, are always in the ratio of small whole numbers. 2.2 The Discovery of Atomic Structure Scientists realized by 1850 that atoms are composed of charged particles. If the subatomic particles that are building blocks for an atom are electrically charged, it must be assumed that like charges will repel each other and opposite charges will attract one another. Cathode Rays and Electrons Thompson’s Cathode Ray Experiment – Fig. 2.4 in text Cathode rays are produced when a high voltage is applied across a tube – negative particles move from the negative electrode (cathode) to the positive electrode (anode) The electron paths can be altered by a magnetic field In 1897 Thompson determined the charge-to-mass ratio of an electron 2
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o Charge – to mass ratio = 1.76 x 10 8 C/g (C = Coulomb, SI unit of charge) Millikan’s Oil Drop Experiment – to determine the charge on the electron in order to find its mass Charge on the electron was found to be equal to 1.6 x 10 -9 C Charge/mass for an electron = 1.76 x 10 8 C/g The mass is therefore equal to = 1.6 x 10 -19 C = 9.10 x 10 -28 g 1.76 x 10 8 C/g Radioactivity The spontaneous emission of radiation is called radioactivity Look at experiment 2.7 in your text Negatively-charge emission (β-radiation) – electrons Neutral radiation (γ-radiation) – similar to x-rays Positively charged radiation (α – radiation) – high mass particles – He
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This note was uploaded on 05/14/2009 for the course CHEM 121 taught by Professor Wyzlouzil during the Fall '07 term at Ohio State.

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121lec2 - 1 Chemistry 121 Chapter 2 ATOMS MOLECULES AND...

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