Lecture%205

Lecture%205 - Lecture 5 C. Compounds C.1 What are...

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Lecture 5
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C. Compounds C.1 What are Compounds? Two or more elements held together by bonds. Law of Constant Proportions : Every molecule or crystal (ionic compounds) of a particular compound contains the same elements in the same ratios. Ratios indicated by subscripts: MgCl 2 , NaHCO 3 . Physical Properties : A compound does not resemble any of the elements that contribute to its structure. e.g. S 8 : yellow powder, O 2 : odorless gas, SO 2 : pungent, poisonous gas. C: black powder (graphite), H 2 : odorless gas, CH 3 OH (methanol): volatile liquid
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Classes of Compounds If only two types of elements: binary compound. Binary Molecular Compounds (Covalent Bonds) : Two nonmetals. e.g. H 2 O, NH 3 , CO 2 Binary Ionic Compounds : Ionic bonds between a metal and a nonmetal: NaCl, CaCl 2, KI Ions Atoms that bear a charge due to the loss or gain of electrons. Monoatomic cations: Na + , Al 3+ , Mg 2+ Monoatomic anions: Cl - , O 2- Polyatomic cation: NH 4 + Polyatomic anion: CO 3 2-
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Types of Formulas for Covalently-bonded Molecules Molecular Formulas: Shows ratio of elements but does not indicate bonding arrangement: ethanol: C 2 H 6 O ammonia: NH 3 glucose: C 6 H 12 O 6
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Structural Formulas (two types): 1) Definitive: Element symbols connected by lines. # lines = # of electron pairs in bond. 2) Condensed: Element symbols and subscripts. Bonding arrangement is implied but not explicitly shown. ethanol: CH 3 CH 2 OH ammonia: NH 3 glucose: CH 2 OHCHOHCHOHCHOHCHOHCHO
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Line Structures: Each line represents a pair of bonded C atoms. (Double line: two electron pairs. Triple line: three pairs). H atoms not shown but assumed present according to the consistent property of each C to form four bonds (by sharing its four valence electrons). Each C—H bond contains one electron pair. Symbols for any other elements in structure are shown.
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Space-filling Model Overlapping spheres, scaled to the size of each type of atom. Good approximation of the actual shape of the molecule. Ball-and-Stick Model Spheres representing atoms not to scale. Shows bond lengths and angles more clearly than space-filling model.
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Density Isosurface Model Similar to space-filling model. Slightly more precise representation of the shape of electron clouds of each atom. Electronic Potential (Elpot) Model: Density isosurface color-coded to show variation in electrical charge: blue = positive, red = negative.
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C.3 Ions and Ionic Compounds Solid form is a crystal; i.e. a lattice of alternating cations and anions. Ionic compounds do not form “molecules”. Ratio of cations to anions in crystal is such that net charge is zero. For diatomic or polyatomic ions, multiple ions indicated by:
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This note was uploaded on 04/24/2008 for the course CHEM 6A taught by Professor Pomeroy during the Fall '08 term at UCSD.

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Lecture%205 - Lecture 5 C. Compounds C.1 What are...

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