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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 Spring '06
 Hooker
 Chemistry, Electron

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