Chemical Bonding II: Molecular Geometry and Hybridization of Atomic Orbitals Chapter 10 Chang & Goldsby Modified by Dr. Juliet Hahn Copyright McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 1
VSEPR: 4 Electron Groups Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 4 4 0 tetrahedral tetrahedral AB 3 E 3 1 tetrahedral trigonal pyramidal Copyright McGraw-Hill Education. Permission required for reproduction or display. End 11/1/17 9 am & 10 am 2
VSEPR: 4 Electron Groups (2) Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 4 4 0 tetrahedral tetrahedral AB 3 E 3 1 tetrahedral trigonal pyramidal AB 2 E 2 2 2 tetrahedral bent Copyright McGraw-Hill Education. Permission required for reproduction or display. 3
VSEPR: 5 Electron Groups Class # of atoms bonded to # lone pairs on Arrangement of electron pairs AB 5 5 0 trigonal AB 4 E 4 1 trigonal Molecular Geometry trigonal distorted tetrahedron see saw Copyright McGraw-Hill Education. Permission required for reproduction or display. 4
VSEPR: 5 Electron Groups (2) Class # of atoms bonded to # lone pairs on Arrangement of electron pairs AB 5 5 0 trigonal AB 4 E 4 1 trigonal AB 3 E 2 3 2 trigonal Molecular Geometry trigonal distorted tetrahedron T-shaped Copyright McGraw-Hill Education. Permission required for reproduction or display. 5
Class VSEPR: 5 Electron Groups (3) # of atoms bonded to # lone pairs on Arrangement of electron pairs AB 5 5 0 trigonal AB 4 E 4 1 trigonal AB 3 E 2 3 2 trigonal AB 2 E 3 2 3 trigonal Molecular Geometry trigonal distorted tetrahedron T-shaped linear Copyright McGraw-Hill Education. Permission required for reproduction or display. 6
Class VSEPR: 6 Electron Groups # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 6 6 0 octahedral octahedral AB 5 E 5 1 octahedral square pyramidal Copyright McGraw-Hill Education. Permission required for reproduction or display. 7
Class VSEPR: 6 Electron Groups (2) # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 6 6 0 octahedral octahedral AB 5 E 5 1 octahedral square pyramidal AB 4 E 2 4 2 octahedral square planar Copyright McGraw-Hill Education. Permission required for reproduction or display. 8
Summary of Molecular Shapes 9
Predicting Molecular Geometry 1. Draw Lewis structure for molecule. 2. Count number of lone pairs on the and number of atoms bonded to the. 3. Use VSEPR to predict the geometry of the molecule. End exam IV 10
Example 10.1 Use the VSEPR model to predict the geometry of the following molecules and ions: (a) AsH 3 (b)of 2 (c) AlCl 4 (d)i 3 (e) C 2 H 4
Example 10.1 (1) Strategy The sequence of steps in determining molecular geometry is as follows: draw Lewis structure find arrangement of electron pairs find arrangement of bonding pairs determine geometry based on bonding pairs Solution (a) The Lewis structure of AsH 3 is There are four electron pairs around the ; therefore, the electron pair arrangement is tetrahedral (see Table 10.1).
Example 10.1 (2) Recall that the geometry of a molecule is determined only by the arrangement of atoms (in this case the As and H atoms). Thus, removing the lone pair leaves us with three bonding pairs and a trigonal pyramidal geometry, like NH 3. We cannot predict the HAsH angle accurately, but we know that it is less than 109.5 because the repulsion of the bonding electron pairs in the As H bonds by the lone pair on As is greater than the repulsion between the bonding pairs. (b) The Lewis structure of OF 2 is There are four electron pairs around the ; therefore, the electron pair arrangement is tetrahedral.
Example 10.1 (3) Recall that the geometry of a molecule is determined only by the arrangement of atoms (in this case the O and F atoms). Thus, removing the two lone pairs leaves us with two bonding pairs and a bent geometry, like H 2 O. We cannot predict the FOF angle accurately, but we know that it must be less than 109.5 because the repulsion of the bonding electron pairs in the O F bonds by the lone pairs on O is greater than the repulsion between the bonding pairs. (c) The Lewis structure of AlCl 4 is
Example 10.1 (4) There are four electron pairs around the ; therefore, the electron pair arrangement is tetrahedral. Because there are no lone pairs present, the arrangement of the bonding pairs is the same as the electron pair arrangement. Therefore, AlCl 4 has a tetrahedral geometry and the ClAlCl angles are all 109.5. (d) The Lewis structure of I 3 is There are five electron pairs around the central I atom; therefore, the electron pair arrangement is trigonal. Of the five electron pairs, three are lone pairs and two are bonding pairs.
Example 10.1 (5) Recall that the lone pairs preferentially occupy the equatorial positions in a trigonal bipyramid (see Table 10.2). Thus, removing the lone pairs leaves us with a linear geometry for, that is, all three I atoms lie in a straight line. (e) The Lewis structure of C 2 H 4 is The C = C bond is treated as though it were a single bond in the VSEPR model. Because there are three electron pairs around each C atom and there are no lone pairs present, the arrangement around each C atom has a trigonal planar shape like BF 3, discussed earlier.
Example 10.1 (6) Thus, the predicted bond angles in C 2 H 4 are all 120. Comment (1) The I 3 ion is one of the few structures for which the bond angle 180 can be predicted accurately even though the contains lone pairs. (2) In C 2 H 4,all six atoms lie in the same plane. The overall planar geometry is not predicted by the VSEPR model, but we will see why the molecule prefers to be planar later. In reality, the angles are close, but not equal, to 120 because the bonds are not all equivalent.