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chem lecture 10

chem lecture 10 - Chemistry 6A 1 How do we correct the...

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Chemistry 6A 1
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Chemistry 6A 2 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 3
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Chemistry 6A 4 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 5 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 6 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 7 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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Chemistry 6A 8 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, electronegativities correlate reasonably well with IEs. That is, the higher IE 1 , the greater the value of χ for an element.
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Chemistry 6A 9 As with IE and EA , values of χ are highest in the upper-right of the periodic table and lowest in the lower left.
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Chemistry 6A 10 Atoms which accept electrons readily have high IE and EA
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