CH 5 - CH 5: The Atom Renee Y. Becker CHM 1025 Valencia...

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1 CH 5: The Atom Renee Y. Becker CHM 1025 Valencia Community College
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2 Dalton Model of the Atom John Dalton proposed that all matter is made up of tiny particles. These particles are molecules or atoms. Molecules can be broken down into atoms by chemical processes. Atoms cannot be broken down by chemical or physical processes.
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3 Dalton’s Model According to the law of definite composition , the mass ratio of carbon to oxygen in carbon dioxide is always the same. Carbon dioxide is composed of 1 carbon atom and 2 oxygen atoms. Similarly, 2 atoms of hydrogen and 1 atom of oxygen combine to give water. Dalton proposed that 2 hydrogen atoms could substitute for each oxygen atom in carbon dioxide to make methane with 1 carbon atom and 4 hydrogen atoms. Indeed, methane is CH 4 !
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4 Dalton’s Theory A Summary of Dalton’s Atomic Theory: 1. An element is composed of tiny, indivisible, indestructible particles called atoms. 2. All atoms of an element are identical and have the same properties. 3. Atoms of different elements combine to form compounds. 4. Compounds contain atoms in small whole number ratios. 5. Atoms can combine in more than one ratio to form different compounds.
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5 Dalton’s Atomic Theory The first two parts of Dalton’s theory were later proven incorrect. We will see this later. Proposals 3, 4, and 5 are still accepted today. Dalton’s theory was an important step in the further development of atomic theory.
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6 Subatomic Particles About 50 years after Dalton’s proposal, evidence was seen that atoms were divisible . Two subatomic particles were discovered. negatively charged electrons , e positively charge protons , p + An electron has a relative charge of -1, and a proton has a relative charge of +1.
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7 Thomson’s Model of the Atom J. Thomson proposed a subatomic model of the atom in 1903. Thomson proposed that the electrons were distributed evenly throughout a homogeneous sphere of positive charge. This was called the “plum pudding” mode of the atom.
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8 Mass of Subatomic Particles Originally, Thomson could only calculate the mass-to-charge ratio of a proton and an electron. Robert Millikan determined the charge of an electron in 1911. Thomson calculated the masses of a proton and electron: an electron has a mass of 9.11 × 10 -28 g a proton has a mass of 1.67 × 10 -24 g
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9 Types of Radiation There are three types of radiation: alpha ( α ), beta ( β γ ) Alpha rays are composed of helium atoms stripped of their electrons (helium nuclei). Beta rays are composed of electrons. Gamma rays are high-energy electromagnetic radiation.
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Rutherford’s Gold Foil Experiment Rutherford’s student fired alpha particles at thin gold foils. If the “plum pudding” model of the atom was correct, α - particles should pass through undeflected. However, some of the
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This note was uploaded on 10/19/2011 for the course CHEM 1025 taught by Professor Smith during the Spring '10 term at Valencia.

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CH 5 - CH 5: The Atom Renee Y. Becker CHM 1025 Valencia...

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