lecture 1.1

lecture 1.1 -...

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Unformatted text preview: 1-1-1ElectronegativityElectronegativity◆Electronegativity:Electronegativity:•a measure of an atom’s attraction for the electrons it shares with another atom in a chemical bond◆Pauling scalePauling scale•generally increases left to right in a row•generally increases bottom to top in a column1-1-2Covalent BondsCovalent Bonds◆The simplest covalent bond is that in H2•the single electrons from each atom combine to form an electron pair•the shared pair functions in two ways simultaneously; it is shared by the two atoms and fills the valence shell of each atom◆The number of shared pairs•one shared pair forms a single bond•two shared pairs form a double bond•three shared pairs form a triple bondHHH-H+∆Η= -435 κϑ(-104 κχαλ29 /μ ολ1-1-3Polar and Nonpolar Covalent BondsPolar and Nonpolar Covalent Bonds◆Although all covalent bonds involve sharing of electrons, they differ widely in the degree of sharing◆We divide covalent bonds into•nonpolar covalent bonds•polar covalent bondsDifference in ElectronegativityBetween Bonded AtomsType of BondLess than 0.50.5 to 1.9Greater than 1.9Nonpolar covalentPolar covalentIons form1-1-4Polar and Nonpolar Covalent BondsPolar and Nonpolar Covalent Bonds•an example of a polar covalent bond is that of H-Cl•the difference in electronegativity between Cl and H is 3.0 - 2.1 = 0.9•we show polarity by using the symbols δδ+ and δδ-, or by using an arrow with the arrowhead pointing toward the negative end and a plus sign on the tail of the arrow at the positive endHClδ+d-HCl1-1-5Polar Covalent BondsPolar Covalent Bonds◆Bond dipole moment (Bond dipole moment (μμ):):•a measure of the polarity of a covalent bond•the product of the charge on either atom of a polar bond times the distance between the nuclei•Table 1.7 shows average bond dipole moments of selected covalent bondsH-OH-NH-CH-SC-IC-FC-ClC-BrC-N-C-OC=NC=O- Bond Dipole (D)Bond1.41.51.51.41.2Bond Bond Dipole (D)1.30.30.70.20.72.33.5 Bond Dipole (D)Bond1-1-6Lewis Structures - Table 1.3Lewis Structures - Table 1.3◆In neutral molecules•hydrogen has one bond•carbon has 4 bonds and no lone pairs•nitrogen has 3 bonds and 1 lone pair•oxygen has 2 bonds and 2 lone pairs•halogens have 1 bond and 3 lone pairsH2O (8)NH3 (8)CH4 (8)HCl (8)C2H4 (12)C2H2 (10)CH2O (12)H2CO3 (24)H-O-HH-N-HHH-C-HHHH-ClH-C C-HHHCOHHCCHHOOCHHOEthyleneHydrogen chlorideM ethaneAmmoniaWaterCarbonic acidFormaldehydeAcetylene1-1-7Formal ChargeFormal Charge◆Formal charge:Formal charge:the charge on an atom in a molecule or a polyatomic ion◆To derive formal charge1. write a correct Lewis structure for the molecule or ion2. assign each atom all its unshared (nonbonding) electrons and one-half its shared (bonding) electrons3. compare this number with the number of valence electrons in the neutral, unbonded atomNumber of valence electrons in the neutral, unbonded atomAll unsharedelectronsOne half of all shared electrons+Formalcharge=1-...
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This note was uploaded on 03/30/2008 for the course CHEM 262 taught by Professor Forbes during the Spring '08 term at UNC.

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lecture 1.1 -...

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