Chapter 9 Molecular Geometries and Bonding Theories

Chapter 9 Molecular Geometries and Bonding Theories

Lecture Presentation Chapter 9 Molecular Geometry and Bonding Theories 2015 Pearson Education, Inc. James F. Kirby Quinnipiac University Hamden, CT Molecular Shapes Lewis Structures show bonding and lone pairs, but do not denote shape.

However, we use Lewis Structures to help us determine shapes. Here we see some common shapes for molecules with two or three atoms connected to a central atom. Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc. What Determines the Shape of a Molecule? Simply put, electron pairs, whether they be

bonding or nonbonding, repel each other. By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule. This is the Valence-Shell Electron-Pair Repulsion (VSEPR) model. 2015 Pearson Education, Inc. Molecular Geometries and Bonding

Theories Electron Domains We can refer to the directions to which electrons point as electron domains. This is true whether there is one or more electron pairs pointing in that direction. The central atom in this molecule, A, has four electron domains. Molecular

Geometries and Bonding Theories 2015 Pearson Education, Inc. Valence-Shell Electron-Pair Repulsion (VSEPR) Model The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them. (The balloon analogy in the figure to the left demonstrates the

maximum distances, which minimize Molecular Geometries repulsions.) and Bonding Theories 2015 Pearson Education, Inc. Electron-Domain Geometries The Table shows the electron-domain geometries for two through six electron domains around a

central atom. To determine the electron-domain geometry, count the total number of lone pairs, single, double, and triple bonds on Molecular Geometries the central atom. and Bonding Theories 2015 Pearson Education, Inc. Molecular Geometries

Once you have determined the electron-domain geometry, use the arrangement of the bonded atoms to determine the molecular geometry. Tables 9.2 and 9.3 show the potential molecular geometries. We will look at each electron domain Molecular to see what molecular geometries are possible. Geometries and Bonding Theories 2015 Pearson Education, Inc. Linear Electron Domain In the linear domain, there is only one molecular geometry: linear.

NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is. 2015 Pearson Education, Inc. Molecular Geometries and Bonding Theories Trigonal Planar Electron Domain There are two molecular geometries: trigonal planar, if all electron domains are bonding, and bent, if one of the domains is a

nonbonding pair. 2015 Pearson Education, Inc. Molecular Geometries and Bonding Theories Tetrahedral Electron Domain There are three molecular geometries: tetrahedral, if all are bonding pairs, trigonal pyramidal, if one is a nonbonding pair, and bent, if there are two nonbonding pairs. 2015 Pearson Education, Inc.

Molecular Geometries and Bonding Theories Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to compress bond angles. Molecular Geometries and Bonding

Theories 2015 Pearson Education, Inc. Multiple Bonds and Bond Angles Double and triple bonds have larger electron domains than single bonds. They exert a greater repulsive force than single bonds, making their bond angles greater. Molecular Geometries and Bonding

Theories 2015 Pearson Education, Inc. Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry: Axial Equatorial Lone pairs occupy equatorial positions. Molecular Geometries and Bonding Theories

2015 Pearson Education, Inc. Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain: Trigonal bipyramidal Seesaw T-shaped Linear 2015 Pearson Education, Inc. Molecular

Geometries and Bonding Theories Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: Octahedral Square pyramidal Square planar Molecular Geometries

and Bonding Theories 2015 Pearson Education, Inc. Shapes of Larger Molecules For larger molecules, look at the geometry about each atom rather than the molecule as a whole. Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc.

Sample Exercise 9.1 Using the VSEPR Model Use the VSEPR model to predict the molecular geometry of (a) O3, (b) SnCl3. Solution Analyze We are given the molecular formulas of a molecule and a polyatomic ion, both conforming to the general formula ABn and both having a central atom from the p block of the periodic table. (Notice that for O3, the A and B atoms are all oxygen atoms.) Plan To predict the molecular geometries, we draw their Lewis structures and count electron domains around the central atom to get the electron-domain geometry. We then obtain the molecular geometry from the arrangement of the domains that are due to bonds. Solve (a) We can draw two resonance structures for O3: Chemistry: The Central Science, 13th Edition Brown/LeMay/Bursten/Murphy/Woodward/Stoltzfus

2015 Pearson Education, Inc. Sample Exercise 9.1 Using the VSEPR Model Continued Because of resonance, the bonds between the central O atom and the outer O atoms are of equal length. In both resonance structures the central O atom is bonded to the two outer O atoms and has one nonbonding pair. Thus, there are three electron domains about the central O atoms. (Remember that a double bond counts as a single electron domain.) The arrangement of three electron domains is trigonal planar (Table 9.1). Two of the domains are from bonds, and one is due to a nonbonding pair. So, the molecular geometry is bent with an ideal bond angle of 120 (Table 9.2). Chemistry: The Central Science, 13th Edition Brown/LeMay/Bursten/Murphy/Woodward/Stoltzfus 2015 Pearson Education, Inc.

Sample Exercise 9.1 Using the VSEPR Model Continued Comment As this example illustrates, when a molecule exhibits resonance, any one of the resonance structures can be used to predict the molecular geometry. (b) The Lewis structure for SnCl3 is Chemistry: The Central Science, 13th Edition Brown/LeMay/Bursten/Murphy/Woodward/Stoltzfus 2015 Pearson Education, Inc. Sample Exercise 9.1 Using the VSEPR Model Continued The central Sn atom is bonded to the three Cl atoms and has one nonbonding pair; thus, we have four electron domains, meaning a tetrahedral electron-domain geometry (Table 9.1) with one vertex occupied by a nonbonding pair

of electrons. A tetrahedral electrondomain geometry with three bonding and one nonbonding domains leads to a trigonal-pyramidal molecular geometry (Table 9.2). Chemistry: The Central Science, 13th Edition Brown/LeMay/Bursten/Murphy/Woodward/Stoltzfus 2015 Pearson Education, Inc. Valence-Bond Theory In Valence-Bond Theory, electrons of two atoms begin to occupy the same space. This is called overlap of orbitals. The sharing of space between two electrons of

opposite spin results in a covalent bond. Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc. Overlap and Bonding Increased overlap brings the electrons and nuclei closer together until a balance is reached between the like charge repulsions and the electron-nucleus

attraction. Atoms cant get too close because the internuclear repulsions get too great. 2015 Pearson Education, Inc. Molecular Geometries and Bonding Theories VSEPR and Hybrid Orbitals

VSEPR predicts shapes of molecules very well. How does that fit with orbitals? Lets use H2O as an example: If we draw the best Lewis structure to assign VSEPR, it becomes bent. If we look at oxygen, its electron configuration is 1s22s22p4. If it shares two electrons to fill its valence shell, they should be in 2p. Wouldnt that make the angle 90? Molecular Geometries Why is it 104.5? and Bonding Theories

2015 Pearson Education, Inc. Hybrid Orbitals Hybrid orbitals form by mixing of atomic orbitals to create new orbitals of equal energy, called degenerate orbitals. When two orbitals mix they create two orbitals; when three orbitals mix, they create three orbitals; etc. Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc.

Besp hybridization When we look at the orbital diagram for beryllium (Be), we see that there are only paired electrons in full sub-levels. Be makes electron deficient compounds with two bonds for Be. Why? sp hybridization (mixing of one s orbital and one p orbital) 2015 Pearson Education, Inc. sp Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. These sp hybrid orbitals have two lobes like a p orbital. One of the lobes is larger and more rounded,

as is the s orbital. Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc. Position of sp Orbitals These two degenerate orbitals would align themselves 180 from each other. This is consistent with the observed geometry of Be compounds (like BeF2) and VSEPR: linear. Molecular Geometries

and Bonding Theories 2015 Pearson Education, Inc. BoronThree Electron Domains 2 Gives sp Hybridization Using a similar model for boron leads to three degenerate sp2 orbitals. Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc.

Carbon: sp Hybridization 3 With carbon, we get four degenerate sp3 orbitals. Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc. Sample Exercise 9.5 Describing the Hybridization of a Central Atom Describe the orbital hybridization around the central atom in NH 2.

Solution Analyze We are given the chemical formula for a polyatomic anion and asked to describe the type of hybrid orbitals surrounding the central atom. Plan To determine the central atom hybrid orbitals, we must know the electron-domain geometry around the atom. Thus, we draw the Lewis structure to determine the number of electron domains around the central atom. The hybridization conforms to the number and geometry of electron domains around the central atom as predicted by the VSEPR model. Solve The Lewis structure is Chemistry: The Central Science, 13th Edition Brown/LeMay/Bursten/Murphy/Woodward/Stoltzfus 2015 Pearson Education, Inc. Sample Exercise 9.5 Describing the Hybridization of a Central Atom

Continued Because there are four electron domains around N, the electron-domain geometry is tetrahedral. The hybridization that gives a tetrahedral electron-domain geometry is sp3 (Table 9.4). Two of the sp3 hybrid orbitals contain nonbonding pairs of electrons, and the other two are used to make bonds with the hydrogen atoms. Chemistry: The Central Science, 13th Edition Brown/LeMay/Bursten/Murphy/Woodward/Stoltzfus 2015 Pearson Education, Inc. What Happens with Water? We started this discussion with H2O and the angle question: Why is it 104.5 instead of 90?

Oxygen has two bonds and two lone pairs four electron domains. The result is sp3 hybridization! 2015 Pearson Education, Inc. Molecular Geometries and Bonding Theories Hybrid Orbital Summary 1) Draw the Lewis structure. 2) Use VSEPR to

determine the electron-domain geometry. 3) Specify the hybrid orbitals needed to accommodate these electron pairs. 2015 Pearson Education, Inc. Molecular Geometries and Bonding Theories Types of Bonds How does a double or triple bond form?

It cant, if we only use hybridized orbitals. However, if we use the orbitals which are not hybridized, we can have a side-ways overlap. Two types of bonds: Sigma () bond Pi () bond Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc. Sigma () and Pi () Bonds Sigma bonds are characterized by

head-to-head overlap. cylindrical symmetry of electron density about the internuclear axis. Pi bonds are characterized by side-to-side overlap. electron density above and below the internuclear axis. 2015 Pearson Education, Inc. Molecular Geometries and Bonding Theories Bonding in Molecules Single bonds are

always -bonds. Multiple bonds have one -bond, all other bonds are -bonds. Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc. Localized or Delocalized Electrons Bonding electrons ( or ) that are specifically shared between two atoms are called localized electrons.

In many molecules, we cant describe all electrons that way (resonance); the other electrons (shared by multiple atoms) are called delocalized electrons. Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc. Benzene The organic molecule benzene (C6H6) has six -bonds and a p orbital on each C atom, which form delocalized bonds using one electron from each p orbital.

Molecular Geometries and Bonding Theories 2015 Pearson Education, Inc.

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