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electronegativity

electronegativity - 598 4— Decreasing electronegativity...

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Unformatted text preview: 598 4— Decreasing electronegativity FIGURE I13 The Pauling electronegativity values Elec- tronegativitv generally increases across a period and decreases down a group. The factor of mm is a canversiOn factor between H and eV (the units Originally used by Pauling). TABLE I3.I The Relationship Between Eleclronega- fivity and Bond Type Electronegativity Difference in the Bond Bonding Atoms Type 2T0 Covillent Intermediate P01“ coveJdent Large Ionic CHAPTER I3 Bonding: General Concepts Increasing electronegatimy is an “average" (actually the geometric mean) of the H—H and X—X bond energies: Expected H—X bond energy = [{H—H bond energy)(X—X bond energy)]”2 The difference (A) between the actual (measured) and expected bond energies is A = (H—X)act _ {H—X)exp If H and X have identical electronegativities, (H—X)alct and {H—X)exp are the same and A is 0. On the other hand, if X has a greater electronegativity than H, the shared electron{s) will tend to be closer to the X atom. The mol- ecule will be polar, with the following charge distribution: H—X 8+ 3— Note that this bond can be viewed as having an ionic as well as a covalent component. The electrostatic attraction between the partially charged H and X atoms will lead to a greater bond strength. Thus {H—X)m will be larger than {H—X)exp. The greater the difference in the electronegativities of the atoms, the greater is the ionic component of the bond and the greater is the value of A. Thus the relative electronegativities of H and X can be assigned from the A values. The actual formula Pauling used to calculate electronegativity (EN) dif- ferences is EN(X) — EN(H) = 0.102% where all bond energies are in units of kjfmol. Pauling then obtained absolute electronegativity values for the elements by assigning a value of 4.0 to fluo- rine {the element with the highest electronegativity). Electronegativity values have been determined by this process for virtu- ally all the elements; the results are given in Fig. 13.3. Note that for the rep- resentative elements, electronegativity generally increases from left to right across a period and decreases down a group. The range of electronegativity values is from 4.0 for fluorine to 0.7 for francium. The relationship between electronegativity and bond type is shown in Table 13.1. For identical atoms (electronegativity difference of zero) the elec- trons in the bond are shared equally and no polarity occurs. When two atoms with widely differing electronegativities interact, electron transfer usually ...
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