Class04

Class04 - Chemistry 83 Formation of Interstellar...

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Chemistry 83 Class04 1 Formation of Interstellar Substances-1 It took 500,000 years for the Universe to cool enough so that neutral atoms could form, but . . . 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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Chemistry 83 Class04 2 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? 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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Chemistry 83 Class04 3 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 How do metal atoms transfer electrons when they combine with non-metal atoms? • Atoms attain IG structures by metal atoms transferring sufFcient 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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Chemistry 83 Class04 4 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 Fxed 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. Substances in which component particles are in 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 - Chemistry 83 Formation of Interstellar...

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