Class04

Class04 - Formation of Interstellar Substances-1 It took...

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Formation of Interstellar Substances-1 It took 500,000 years for the Universe to cool enough so that neutral atoms could form, but . . .
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How stable are neutral atoms? All neutral atoms are unstable, i.e., tend to react, except for those of the inert gases The inert gases are members of family 8A so all have 8 valence electrons in their atoms except He (2e) Inert gas rule : atoms combine by rearranging their valence electrons until each atom attains the stable electron structure of the inert gas that is nearest to it in atomic number
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1A 2A 3A 4A 5A 6A 7A 8A 1e 2e 2e for all B-Group elements 3e 4e 5e 6e 7e 8e* *Except He which has but 2e A-Group Numbers (1A-8A) = numbers of valence electrons (1e-8e) Ten B families How many valence electrons do neutral atoms have originally?
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Staley, Essentials of Chemistry, Benjamin Cummings, CA, 1984, 318 *He is an exception * Lewis Dot Symbols represent valence electrons by dots around the chemical symbol of an element How are original valence electrons shown with Lewis symbols?
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How do A-group atoms rearrange their valence electrons to obtain inert gas structures? 8 e* Metal with a non- metal: transfer valence electron(s) from metal atom(s) to non-metal atom(s) Non-metals alone: share pair(s) of valence electrons between two non- metal atoms Metals alone: delocalize valence electron(s) from metal atoms * 2e for H and Li
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How do metal atoms transfer electrons when they combine with non-metal atoms? Atoms attain IG structures by metal atoms transferring sufficient valence electrons to non- metal atoms to produce positive ions (cations) and negative ions (anions) What force holds the particles together? Ionic bond - strong attraction between oppositely charged ions - called ion-ion force or electrostatic force What type of structure results? cations and anions in a repeating structure that has a variable total number of ions, but with cations and anions in a fixed ratio that maintains electrical neutrality
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http://www.geog.ouc.bc.ca/physgeog/contents/6a.html What is an example? Table salt How is an ionic substance represented symbolically ? By an empirical formula - the subscripts in the formula indicate the simplest fixed ratio in which the ions combine to attain electrical neutrality: NaCl Ionic bond Exercise: empirical formula for union of Li with N atoms? Answer on slide 22.
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Substances in which component particles are in a repeating pattern are said to be crystalline substances Robinson, et al., Chemistry: Concepts and Models, Heath, MA, 1992, 316 The smallest unit containing the repeating pattern is called a unit cell Crystal lattice All ionic substances are crystalline and are represented by empirical formulas What are crystalline substances?
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How do metal atoms delocalize their valence electrons when they combine with other metal atoms? Atoms a
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This note was uploaded on 10/19/2011 for the course CHEM 83 taught by Professor Bonk,j during the Fall '08 term at Duke.

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Class04 - Formation of Interstellar Substances-1 It took...

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