vsepr model valence shell electron pair repulsion model a model for predicting the shapes of...

VSEPR MODELValence Shell Electron Pair Repulsion Model

A model for predicting the shapes of molecules and ions in which valence shell electron pairs are arranged about each atom so that electron pair repulsion is minimized.

ELECTRONIC GEOMETRYThe general shape of a molecule determined by the number of electron pairs around the central atom occupying different quadrants. Gives starting point for bond angle.

MOLECULAR GEOMETRYThe general shape of a molecule determined by the relative positions of the atomic nuclei. The nonbonding electron pairs modifiy the geometry.

VSEPR MODELVSEPR MODELI. Draw the Lewis dot structure.

II. Determine the electronic geometryelectronic geometry by counting the number of pairs of electrons around the central atom occupying different quadrants (top, bottom, left, right). This geometry gives the initial bond angle.

Pairs of e- geometry bond angle 2 linear 180o

3 trigonal planar 120o

4 tetrahderal 109.5o

VSEPR MODELVSEPR MODEL

III. Next, using the electronic geometry, determine the number of bonding and nonbonding electron pairs then arrange the electron pairs as far apart as possible.

______ nonbonding pairs require more space than bonding pairs. nonbonding pairs require more space than bonding pairs.______ multiple bonds require more space than single bonds. multiple bonds require more space than single bonds.

IV. The direction in space of the bonding pairs give the molecular geometrymolecular geometry modified by the position of the nonbonding pairs.

Fig. 13-1, p. 369

Fig. 13-2, p. 369

Fig. 13-3, p. 370

Table describing Molecular Geometry

VSPER Theory Number of Number of electronic electronic bonding bonding nonbonding nonbonding molecular moleculare- pairs geometry e- pairs e- pairs geometrye- pairs geometry e- pairs e- pairs geometry

2 linear linear 2 0 linear

3 trigonal planartrigonal planar 3 0 trigonal planar

3 trigonal planartrigonal planar 2 1 bent

4 tetrahedral tetrahedral 4 0 tetrahedral

4 tetrahdral tetrahdral 3 1 trigonal pyramidal

4 tetrahedral tetrahedral 2 2 bent

Fig. 13-4, p. 370

Fig. 13-5, p. 372

Table 13-2, p. 371

p. 374

p. 374

p. 375

p. 375

p. 375

p. 376

Predict the Molecular Geometry for the following molecules.Predict the Molecular Geometry for the following molecules.

1. H2O There are 4 pairs of electrons around the central atom so the electronic geometry is tetrahedraltetrahedral with a bond angle of 109.5o. Since 2 of the pairs of electrons are bonding and the other two pairs are nonbonding, the electronic geometry has been modified to a molecular geometry of BENTBENT.

2. CO2

There are four pairs of electrons in two different quadrents (left & right) so the molecular geometry is LINEAR LINEAR with a bond angle of 180o. . . . .. . . .

: O = C = O :: O = C = O :

Predict the Molecular Geometry for the following molecules.Predict the Molecular Geometry for the following molecules.

3. BH3 There are 3 pairs of electrons around the central atom so the electronic geometry is trigonal planartrigonal planar with a bond angle of 120o. Since all three pairs of electrons are bonding, the electronic geometry has not been modified.

4. NH3

There are 4 pairs of electrons around the central atom so the electronic geometry is tetrahedraltetrahedral with a bond angle of 109.5o. Since 3 of the pairs of electrons are bonding and the other pair is nonbonding, the electronic geometry has been modified to a molecular geometry of TRIGONAL PYRAMIDALTRIGONAL PYRAMIDAL.

Practice ProblemsPractice Problems• Draw the Lewic structure for the following, then predict

both the electronic & molecular geometry. Give the

approximate bond angle. (See instructor for answers)

a) GeH2 b) AsF3 c) AlF3

d) SO2 e) SO3 f) SO32-

g) SiF4 h) C2H4 I) Cl2O

Group Study ProblemsGroup Study Problems Draw the Lewic structure for the following, then

predict both the electronic & molecular geometry.

Give the approximate bond angle.

a) H2S b) PH3 c) CH2O

d) NO2- e) H3PO4 f) CBr4

g) CH2FCl h) C2H2 I) O3