Chemistry - A Molecular Science

Chapter 6 Molecular Structure & Bonding

6.1

MOLECULAR SHAPES

Just as a two-dimensional blueprint provides information about a three-dimensional building, the Lewis structure of a molecule provides information about the three-dimensional structure of a molecule. The transition from a two- to a three-dimensional structure is accomplished with the

valence-s

hell

electron-

pair

repulsion (VSEPR) model.

VSEPR is based on the premise that the regi

ons of negative charge around an atom adopt

positions that minimize the repulsions between them.

Our discussion uses the general terms ‘electron groups’ and ‘electron regions’ rather

than ‘electron pairs’ in describing the results of VSEPR. An electron group or region can be a lone pair*, a single bond, a double bond, or a triple bond. A lone pair and a single bond each consist of a single electron pair a

nd constitute a single electron region. A double

bond consists of two electron pairs, but both

pairs lie in the region between the two bound

nuclei, so they cannot move away from one a

nother. Consequently, the two electron pairs

in a double bond represent a single electron region that minimizes its interactions with other electron regions. Similarly, the three electron pairs of a triple bond represent a single electron region because the three pairs cannot move apart.

Atoms obeying the octet rule can have only tw

o, three, or four electron groups, and

Figure 6.1 shows the orientations of the

groups as predicted by VSEPR. Two electron

groups are oriented such that the angle between them is 180

o (Figure 6.1a). Three electron

groups reside in a plane with an angle of 120

o between them (Figure 6.1b). Four electron

groups disperse themselves in the directions shown in Figure 6.1c. The angle between any two groups in the four-electron group arrangement is 109

o.

The angles between the groups shown in Figure 6.1 assume that the electron groups

are all the same. When the electron groups su

rrounding a central atom are not identical,

the interactions between the groups are not

identical, and the angles deviate somewhat

from those given in the figure. The rela

tive strengths of the interactions are

lone pair-lone pair > lone pair-bondi

ng pair > bonding pair-bonding pair

The

bond angles

, which are the angles formed by th

e intersection of the bonds, are an

important characteristic of molecular shape,

and the above considerations help us to

predict them. The bond angles are generally reduced from the values given in Figure 6.1 by the presence of lone pairs because the inte

raction between lone

pairs and bonding pairs

is stronger than between bonding pairs. As a result, the bonding pairs move away from the lone pairs by moving closer to

one another. The deviation

from the angles shown in Figure

* In molecules with an odd number of valence electrons, a single

electron rather than a lone pair may occupy an electron region.

180

o

120

o

109

o

(a)

(b)

(c)

Figure 6.1 Distributions of electron groups that minimize electron-electron repulsions

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