Chapter2 - CHAPTER 2 Atoms Molecules Ions Atomic Theory of...

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CHAPTER 2 Atoms, Molecules & Ions
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Atomic Theory of Matter The Greek philosopher Democritus (ca 400 BC) proposed that matter could not be continuously sub-divided – that there was a final indivisible particle (Greek – atomos ). This idea was supported by Plato and Aristotle and, later, by the Roman author, Lucretius ( De Rerum Natura )
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Atomic Theory of Matter The theory that atoms are the fundamental building blocks of matter re-emerged in the early 19th century, championed by John Dalton.
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Dalton’s Postulates Each element is composed of extremely small particles called atoms. (Scientists discovered later that atoms are composed of even smaller components called subatomic particles, which we now know as protons, neutrons, and electrons.)
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Dalton’s Postulates All atoms of a given element are identical to one another in mass and properties, but atoms of one element are different from atoms of other elements. (It was discovered later that a given element has different isotopes. )
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Dalton’s Postulates Atoms of an element are not changed into atoms of a different element by chemical reactions. Atoms are neither created nor destroyed in chemical reactions. (They are re-arranged.) (But – it was discovered later that atoms can be interconverted in nuclear reactions!)
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Dalton’s Postulates Compounds are formed when atoms of more than one element combine. A given compound always has the same relative number and kind of atoms.
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Law of Constant Composition Joseph Proust (1754–1826) Also known as the Law of Definite Proportions. The elemental composition of a pure substance never varies.
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Law of Conservation of Mass The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place. 2 H 2 ( g ) + O 2 ( g ) 2 H 2 O ( l )
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Law of Multiple Proportions If two elements, A and B, combine to form more than one compound, then the masses of B which can combine with a fixed amount of A are in the ratio of small whole numbers.
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Law of Multiple Proportions (Example 1) Example: hydrogen ( H ) and oxygen ( O ) can combine to form either water or hydrogen peroxide. When water is formed: 8.0 g O combines with 1.0 g of H When hydrogen peroxide is formed: 16.0 g O combines with 1.0 g of H . Therefore, we know that hydrogen peroxide ( H 2 O 2 ) contains twice as much O as water ( H 2 O ).
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Law of Multiple Proportions (Example 2) Sulfur can combine with oxygen to form two different compounds: Compound A: 1.0 g S + 1.0 g O Compound B: 1.0 g S + 1.5 g O What can conclusions can we make about These two compounds?
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Law of Multiple Proportions (Example 3)
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Streams of negatively charged particles (“ cathode rays ” = electrons ) were found to emanate from cathode tubes. J. J. Thompson is credited with the discovery of the
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This note was uploaded on 04/08/2008 for the course CHE 101 taught by Professor Churchhill during the Spring '08 term at SUNY Buffalo.

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Chapter2 - CHAPTER 2 Atoms Molecules Ions Atomic Theory of...

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