SB_Chem6A_08_Lecture10

SB_Chem6A_08_Lecture10 - The nitrate ion can be described...

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Chemistry 6A The nitrate ion can be described in the Lewis formulation by a set of structures: The set means to BEST represent the nitrate ion as … (a) Two N-to-O single bonds and a third N-O bond that is double (b) Three independent forms that co-exist in equilibrium (c) Three forms in which the electrons must be rapidly exchanging (d) Only one form which is a composite or average of the three structures shown (e) None of the above 1
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Chemistry 6A Which of the following would be properly classified as a set of covalent molecules? (a) NaCl, CH 4 , S 8 (b) NaClO 4 , C 4 H 10 , NH 3 (c) CO 2 , HCN, O 2 (d) CO, NH 4 Cl, C 2 H 6 (e) AgCl, ScF 3 , Cl 2 2
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Chemistry 6A 3 How do we correct the covalent and ionic models? How are bond energies and bond lengths measured and related? Beyond connectivity, what are the spatial arrangements of atoms in a molecule?
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Chemistry 6A 4
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Chemistry 6A 5 In most heteronuclear covalent bonds, the nucleus of one atom attracts e - more strongly than the nucleus of the other atom. This creates an electric dipole moment, μ (Greek mu), whereby one nucleus has a partial (+) charge and the other a partial (–) charge. Partial charges are indicated by superscript δ + or δ - (Greek delta ); The cross-base arrow points to the negative partial charge
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Chemistry 6A 6 A measure of the ability or tendency of an atom to attract electrons from another atom to which it is bonded is termed electronegativity, χ (Greek “chi”). This unitless property is related to the ionization energy and electron affinity , which provide an indication of how readily an atom may want to give up or accept an electron, respectively. When comparing two elements, that with the higher χ value will attract e - more strongly in the bond (it has the greater pulling power).
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Chemistry 6A 7 A covalent bond in which electrons are not shared equally between two atoms is called a polar covalent bond. Nonpolar bonds connect atoms of the same electronegativity. The greater the difference in electronegativity ( Δ EN) between two bonded atoms, the more polar the bond.
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Chemistry 6A 8 Linus Pauling advanced an electronegativity scale which is based on the dissociation energies, D (in eV), of the homonuclear (A-A, B-B) and heteronuclear (A-B) bonds. He defined the difference in electronegativities of two elements A and B as: Pauling argued that the excess bond energy is a result of the ionic component of the bond caused by partial charges on atoms A and B. Electronegativities (unitless averages) computed with this method are based on fluorine having an arbitrarily set value of 4.00 (today χ F = 3.98). The relative values for all other elements are positive < 3.98. χ A − χ B = 0.102 D ( A B ) 0.5( D ( B B ) + D ( B B ) )
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9 Many other electronegativity scales exist, but a simple and intuitive relationship was devised by Robert Mulliken: χ = 1 2 ( IE 1 + EA 1 ) Since ionization energies are much greater than electron affinities,
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This note was uploaded on 12/07/2009 for the course CHEM CHEM 6A taught by Professor Czarkowski during the Fall '07 term at UCSD.

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SB_Chem6A_08_Lecture10 - The nitrate ion can be described...

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