Molecular Geometry and Bonding Theories

Mr. Matthew Totaro AP Chemistry Legacy High School © 2012 Pearson Education, Inc.

Molecular Shapes • The shape of a molecule plays an important role in its reactivity. • By noting the number of bonding and nonbonding electron pairs, we can easily predict the shape of the molecule.

Molecular Geometries and Bonding © 2012 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. © 2012 Pearson Education, Inc.

Molecular Geometries and Bonding

Electron Domains

• The central atom in this molecule, A, has four electron domains.

• We can refer to the electron pairs as electron domains. • In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain.

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Molecular Geometries and Bonding

Valence-Shell Electron-Pair Repulsion Theory (VSEPR)

“The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.” Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Electron-Domain Geometries Table 9.1 contains the electron-domain geometries for two through six electron domains around a central atom.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Electron-Domain Geometries • All one must do is count the number of electron domains in the Lewis structure. • The geometry will be that which corresponds to the number of electron domains. © 2012 Pearson Education, Inc.

Molecular Geometries and Bonding

Molecular Geometries

• The electron-domain geometry is often not the shape of the molecule, however. • The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs. © 2012 Pearson Education, Inc.

Molecular Geometries and Bonding

Molecular Geometries

Within each electron domain, then, there might be more than one molecular geometry. Molecular Geometries and Bonding © 2012 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.

Molecular Geometries and Bonding

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Trigonal Planar Electron Domain

• There are two molecular geometries:

– Trigonal planar, if all the electron domains are bonding, – Bent, if one of the domains is a nonbonding pair. © 2012 Pearson Education, Inc.

Molecular Geometries and Bonding

Nonbonding Pairs and Bond Angle • Nonbonding pairs are physically larger than bonding pairs. • Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Multiple Bonds and Bond Angles • Double and triple bonds place greater electron density on one side of the central atom than do single bonds. • Therefore, they also affect bond angles. Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Tetrahedral Electron Domain

• There are three molecular geometries:

– Tetrahedral, if all are bonding pairs, – Trigonal pyramidal, if one is a nonbonding pair, – Bent, if there are two nonbonding pairs. © 2012 Pearson Education, Inc.

Molecular Geometries and Bonding

Trigonal Bipyramidal Electron Domain • There are two distinct positions in this geometry: – Axial – Equatorial

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Trigonal Bipyramidal Electron Domain

Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.

Molecular Geometries and Bonding

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Trigonal Bipyramidal Electron Domain • There are four distinct molecular geometries in this domain: – – – –

Trigonal bipyramidal Seesaw T-shaped Linear Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

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 © 2012 Pearson Education, Inc.

Predicting the Shapes Around Central Atoms 1. Draw the Lewis structure

2. Determine the number of electron groups around the central atom 3. Classify each electron group as bonding or lone pair, and count each type –

remember, multiple bonds count as one group

4. Use cheat sheet to determine the shape and bond angles Molecular Geometries and Bonding © 2012 19 Pearson Education, Inc.

Example: Predict the geometry and bond angles of PCl3 1. Draw the Lewis structure a) 26 valence electrons

2. Determine the Number of electron groups around central atom a) four electron groups around P Tro: Chemistry: A Molecular Approach, 2/e 20

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Molecular Geometries and Bonding

Example: Predict the geometry and bond angles of PCl3 3. Classify the electron groups a) three bonding groups b) one lone pair

4. Use Table 10.1 to determine the shape and bond angles a) four electron groups around P = tetrahedral electron geometry b) three bonding + one lone pair = trigonal pyramidal molecular geometry c) trigonal pyramidal = bond angles less than 109.5° Tro: Chemistry: A Molecular Approach, 2/e

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Molecular Geometries and Bonding

Practice – Predict the molecular geometry and bond angles in SiF5−

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Practice – Predict the molecular geometry and bond angles in SiF5─ Si least electronegative 5 electron groups on Si

Si is central atom Si = 4e─ F5 = 5(7e─) = 35e─ (─) = 1e─ total = 40e─

5 bonding groups 0 lone pairs

Shape = trigonal bipyramid Bond angles Feq–Si–Feq = 120° Feq–Si–Fax = 90° Molecular Geometries and Bonding

Tro: Chemistry: A Molecular Approach, 2/e

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Practice – Predict the molecular geometry and bond angles in ClO2F

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Practice – Predict the molecular geometry and bond angles in ClO2F Cl least electronegative 4 electron groups on Cl

Cl is central atom Cl = 7e─ O2 = 2(6e─) = 12e─ F = 7e─ Total = 26e─

3 bonding groups 1 lone pair Shape = trigonal pyramidal Bond angles O–Cl–O < 109.5° O–Cl–F < 109.5° Molecular Geometries and Bonding

Tro: Chemistry: A Molecular Approach, 2/e

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Representing 3-Dimensional Shapes on a 2-Dimensional Surface • One of the problems with drawing molecules is trying to show their dimensionality • By convention, the central atom is put in the plane of the paper • Put as many other atoms as possible in the same plane and indicate with a straight line • For atoms in front of the plane, use a solid wedge • For atoms behind the plane, use a hashed Molecular wedge Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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SF6 F F

F S

F

F F

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole. Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Multiple Central Atoms

• Many molecules have larger structures with many interior atoms • We can think of them as having multiple central atoms • When this occurs, we describe the shape around each central atom in sequence • •

shape around left C is tetrahedral shape around center C is trigonal planar shape around right O is tetrahedral-bent

H

O

• •

| || • • H − C − C − O − H | • • H Molecular Geometries and Bonding

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Describing the Geometry of Methanol

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Describing the Geometry of Glycine

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Practice – Predict the molecular geometries in H3BO3

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Practice – Predict the molecular geometries in H3BO3 oxyacid, so H attached to O 34 electron electron groups groups on on B O

B least electronegative

O B has has 2 3 bonding groups 2 0 lone ponepairs pairs

B Is Central Atom B = 3e─ O3 = 3(6e─) = 18e─ H3 = 3(1e─) = 3e─ Total = 24e─

Shape on B = trigonal planar Shape on O = tetrahedral bent Molecular Geometries and Bonding

Tro: Chemistry: A Molecular Approach, 2/e

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Molecular Polarity

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Polarity • In Chapter 8, we discussed bond dipoles. • But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Polarity

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Molecule Polarity

The O─C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule. Tro: Chemistry: A Molecular Approach, 2/e

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Molecular Geometries and Bonding

Molecule Polarity

The H─O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule. © 2012 40 Pearson Education, Inc.

Molecular Geometries and Bonding

Predicting Polarity of Molecules 1. Draw the Lewis structure and determine the molecular geometry 2. Determine whether the bonds in the molecule are polar a) if there are not polar bonds, the molecule is nonpolar

3. Determine whether the polar bonds add together to give a net dipole moment

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Example: Predict whether NH3 is a polar molecule 1. Draw the Lewis structure and determine the molecular geometry a) eight valence electrons b) three bonding + one lone pair = trigonal pyramidal molecular geometry Tro: Chemistry: A Molecular Approach, 2/e

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Molecular Geometries and Bonding

Example: Predict whether NH3 is a polar molecule 2. Determine if the bonds are polar a) electronegativity difference b) if the bonds are not polar, we can stop here and declare the molecule will be nonpolar

ENN = 3.0 ENH = 2.1 3.0 − 2.1 = 0.9 therefore the bonds are polar covalent Molecular Geometries and Bonding

Tro: Chemistry: A Molecular Approach, 2/e

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Example: Predict whether NH3 is a polar molecule 3) Determine whether the polar bonds add together to give a net dipole moment a) vector addition b) generally, asymmetric shapes result in uncompensated polarities and a net dipole moment

The H─N bond is polar. All the sets of bonding electrons are pulled toward the N end of the molecule. The net result is a polar Molecular molecule. Geometries and Bonding

Tro: Chemistry: A Molecular Approach, 2/e

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Practice – Decide whether the following molecules are polar EN O = 3.5 N = 3.0 Cl = 3.0 S = 2.5

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Practice – Decide whether the following molecules are polar

Trigonal Bent

Trigonal Planar 2.5

1. polar bonds, N-O 2. asymmetrical shape polar

1. polar bonds, all S-O 2. symmetrical shape nonpolar Molecular Geometries and Bonding

Tro: Chemistry: A Molecular Approach, 2/e

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Molecular Polarity Affects Solubility in Water • Polar molecules are attracted to other polar molecules • Because water is a polar molecule, other polar molecules dissolve well in water – and ionic compounds as well

• Some molecules have both polar and nonpolar parts © 2012 47 Pearson Education, Inc.

Molecular Geometries and Bonding

Orbital Hybridization

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Overlap and Bonding • We think of covalent bonds forming through the sharing of electrons by adjacent atoms. • In such an approach this can only occur when orbitals on the two atoms overlap. Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Overlap and Bonding • Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron– electron repulsion. • However, if atoms get too close, the internuclear repulsion greatly raises the energy. © 2012 Pearson Education, Inc.

Molecular Geometries and Bonding

Hybrid Orbitals • Consider beryllium:

– In its ground electronic state, beryllium would not be able to form bonds, because it has no singly occupied orbitals.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Hybrid Orbitals But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Hybrid 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 © 2012 Pearson Education, Inc.

Hybrid Orbitals • These two degenerate orbitals would align themselves 180° from each other. • This is consistent with the observed geometry of beryllium compounds: linear.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Unhybridized C Orbitals Predict the Wrong Bonding & Geometry

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Methane Formation with sp3 C

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Ammonia Formation with sp3 N

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Example: Predict the hybridization and bonding scheme for CH3CHO Determine the hybridization of the interior atoms

C1 = tetrahedral ∴ C1 = sp3 C2 = trigonal planar ∴ C2 = sp2

Sketch the molecule and orbitals

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Example: Predict the hybridization and bonding scheme for CH3CHO Label the bonds

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Practice – Predict the hybridization of all the atoms in H3BO3

Molecular Geometries and Bonding Tro: Chemistry: A Molecular Approach, 2/e

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Practice – Predict the hybridization and bonding scheme of all the atoms in NClO

•• •O •

•• N

•• Cl •• ••

σ:Osp2─Nsp2

↑↓

↑↓

O ↑↓ N

↑↓

↑↓

Cl

↑↓

O

N

π:Op─Np Tro: Chemistry: A Molecular Approach, 2/e

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σ:Nsp2─Clp

Cl

Molecular Geometries and Bonding

Valence Bond Theory

• Hybridization is a major player in this approach to bonding. • There are two ways orbitals can overlap to form bonds between atoms.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Sigma (σ) Bonds

• Sigma bonds are characterized by

– Head-to-head overlap. – Cylindrical symmetry of electron density about the internuclear axis. Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Pi (π) Bonds

• Pi bonds are characterized by

– Side-to-side overlap. – Electron density above and below the internuclear axis. Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Single Bonds Single bonds are always σ bonds, because σ overlap is greater, resulting in a stronger bond and more energy lowering.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Multiple Bonds In a multiple bond, one of the bonds is a σ bond and the rest are π bonds.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Multiple Bonds

• In a molecule like formaldehyde (shown at left), an sp2 orbital on carbon overlaps in σ fashion with the corresponding orbital on the oxygen. • The unhybridized p orbitals overlap in π fashion.

Molecular Geometries and Bonding

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Multiple Bonds In triple bonds, as in acetylene, two sp orbitals form a σ bond between the carbons, and two pairs of p orbitals overlap in π fashion to form the two π bonds.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Molecular Geometries and Bonding © 2012 69 Pearson Education, Inc.

Delocalized Electrons: Resonance When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Delocalized Electrons: Resonance • In reality, each of the four atoms in the nitrate ion has a p orbital. • The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Delocalized Electrons: Resonance This means the π electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Resonance The organic molecule benzene has six σ bonds and a p orbital on each carbon atom.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.

Resonance • In reality the π electrons in benzene are not localized, but delocalized. • The even distribution of the π electrons in benzene makes the molecule unusually stable.

Molecular Geometries and Bonding © 2012 Pearson Education, Inc.