Ch-8 Part II Bonding: General Concepts

Molecular Geometry and Bond Theory In this chapter we will discuss the geometries of molecules in terms of their electronic structure –. We will also explore two theories of chemical bonding: valence bond theory and molecular orbital theory. Molecular geometry is the general shape of a molecule, as determined by the relative positions of the atomic nuclei- three dimensional arrangement of atoms in a molecule

The Valence-Shell Electron Pair Repulsion Model The valence-shell electron pair repulsion (VSEPR) model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible. To predict the relative positions of atoms around a given atom using the VSEPR model, you first note the arrangement of the electron pairs around that central atom.

Predicting Molecular Geometry The following rules and figures will help discern electron pair arrangements. 1.Draw the Lewis structure 2.Determine how many electrons pairs are around the central atom. Count a multiple bond as one pair. 3.Arrange the electrons pairs.

Arrangement of Electron Pairs About an Atom 3 pairs Trigonal planar 2 pairs Linear 4 pairs Tetrahedral 5 pairs Trigonal bipyramidal 6 pairs Octahedral

Valence shell electron pair repulsion (VSEPR) model: Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs. AB 2 20 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry linear B B

Cl Be 2 atoms bonded to central atom 0 lone pairs on central atom

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar AB 4 40 tetrahedral

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar AB 4 40 tetrahedral AB 5 50 trigonal bipyramidal trigonal bipyramidal

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar AB 4 40 tetrahedral AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 6 60 octahedral

bonding-pair vs. bonding pair repulsion (a) lone-pair vs. lone pair Repulsion (c) lone-pair vs. bonding pair repulsion (b) c b a ‹‹

18 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar AB 2 E21 trigonal planar bent

19 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 E31 AB 4 40 tetrahedral trigonal pyramidal

20 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 4 40 tetrahedral AB 3 E31tetrahedral trigonal pyramidal AB 2 E 2 22tetrahedral bent

21 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 4 E41 trigonal bipyramidal distorted tetrahedron

22 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 4 E41 trigonal bipyramidal distorted tetrahedron AB 3 E 2 32 trigonal bipyramidal T-shaped

23 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 4 E41 trigonal bipyramidal distorted tetrahedron AB 3 E 2 32 trigonal bipyramidal T-shaped AB 2 E 3 23 trigonal bipyramidal linear

24 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 6 60 octahedral AB 5 E51 octahedral square pyramidal

25 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 6 60 octahedral AB 5 E51 octahedral square pyramidal AB 4 E 2 42 octahedral square planar

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Dipole Moment and Molecular Geometry The dipole moment is a measure of the degree of charge separation in a molecule. We can view the polarity of individual bonds within a molecule as vector quantities. Thus, molecules that are perfectly symmetric have a zero dipole moment. These molecules are considered nonpolar.   

Dipole Moments and Polar Molecules H F electron rich region electron poor region    = Q x r Q is the charge, only the magnitude, not to its sign. r is the distance between charges 1 D (D=debye unit) = 3.36 x C m (C is coulomb, m is meter

No electric field Electric field

Dipole Moment and Molecular Geometry Molecules that exhibit any asymmetry in the arrangement of electron pairs would have a nonzero dipole moment. These molecules are considered polar.   H N HH :  

The arrow shows the shift of electron density from less electronegative atom to the more electronegative atom Bond moment (B.M) is the individual dipole moment (D.M) which is a vector quantity, has both magnitude and direction. D.M = sum of vector B.M

Which of the following molecules have a dipole moment? H 2 O, CO 2, SO 2, and CH 4 O H H dipole moment polar molecule S O O CO O no dipole moment nonpolar molecule dipole moment polar molecule C H H HH no dipole moment nonpolar molecule

Does BF 3 have a dipole moment? The trigonal planar shape means that the three bond moments exactly cancel one another, makes it a nonpolar molecule.

Does CH 2 Cl 2 have a dipole moment?

10.2 Because chlorine is more electronegative than carbon, which is more electronegative than hydrogen, the bond moments do not cancel and the molecule possesses a dipole moment: Thus, CH 2 Cl 2 is a polar molecule.

Predicting Molecular Geometry 1.Draw Lewis structure for molecule. 2.Count number of lone pairs on the central atom and number of atoms bonded to the central atom. 3.Use VSEPR to predict the geometry of the molecule. What are the molecular geometries of SO 2 and SF 4 ? SO O AB 2 E bent S F F F F AB 4 E distorted tetrahedron

Predicting Molecular Geometry Two electron pairs (linear arrangement). You have two double bonds, or two electron groups about the carbon atom. Thus, according to the VSEPR model, the bonds are arranged linearly, and the molecular shape of carbon dioxide is linear. Bond angle is 180 o. : : : :

Predicting Molecular Geometry Three electron pairs (trigonal planar arrangement). The three groups of electron pairs are arranged in a trigonal plane. Thus, the molecular shape of COCl 2 is trigonal planar. Bond angle is 120 o. Cl C : : : O : : : ::

Predicting Molecular Geometry Three electron pairs (trigonal planar arrangement). Ozone has three electron groups about the central oxygen. One group is a lone pair. These groups have a trigonal planar arrangement. OO O : : : : : :

Predicting Molecular Geometry Three electron pairs (trigonal planar arrangement). Since one of the groups is a lone pair, the molecular geometry is described as bent or angular. OO O : : : : : :

Predicting Molecular Geometry Three electron pairs (trigonal planar arrangement). Note that the electron pair arrangement includes the lone pairs, but the molecular geometry refers to the spatial arrangement of just the atoms. OO O : : : : : :

Predicting Molecular Geometry Four electron pairs (tetrahedral arrangement). Four electron pairs about the central atom lead to three different molecular geometries. : Cl : : : :Cl : : CCl: : : H N H H : : O H H :

Predicting Molecular Geometry Four electron pairs (tetrahedral arrangement). : Cl : : : : Cl : : C H N H H : : O H H : tetrahedral Cl : : :

Predicting Molecular Geometry Four electron pairs (tetrahedral arrangement). : Cl : : : : Cl : : C : O H H : tetrahedral Cl : : : H N HH : trigonal pyramid

Predicting Molecular Geometry Four electron pairs (tetrahedral arrangement). : Cl : : : : Cl : : C : O H H : tetrahedral Cl : : : trigonal pyramidbent H N HH :

Predicting Molecular Geometry Five electron pairs (trigonal bipyramidal arrangement). This structure results in both 90 o and 120 o bond angles. : F : : : F : : : : F : : P F : : :

Predicting Molecular Geometry Other molecular geometries are possible when one or more of the electron pairs is a lone pair. SF 4 ClF 3 XeF 2 Let’s try their Lewis structures.

Predicting Molecular Geometry Other molecular geometries are possible when one or more of the electron pairs is a lone pair. S ClF 3 XeF 2 F F F F see-saw :

Predicting Molecular Geometry Other molecular geometries are possible when one or more of the electron pairs is a lone pair. XeF 2 see-saw S F F F F : Cl F F : : F T-shape

Predicting Molecular Geometry Other molecular geometries are possible when one or more of the electron pairs is a lone pair. see-saw S F F F F : Cl F F : : F T-shape Xe F F : : : linear

Predicting Molecular Geometry Six electron pairs (octahedral arrangement). This octahedral arrangement results in 90 o bond angles. F:F: : : : F : : S F:F: : : : : :F: : :

Predicting Molecular Geometry Six electron pairs (octahedral arrangement). Six electron pairs also lead to other molecular geometries. IF 5 XeF 4

Predicting Molecular Geometry Six electron pairs (octahedral arrangement). XeF 4 I F F F : F F square pyramid

Predicting Molecular Geometry Six electron pairs (octahedral arrangement). I F F F : F F square pyramid Xe F F : F F : square planar