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Chapter 9 – Molecular Geometry and Bond Theory
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VSEPR Theory Draw the Lewis structure.
Predict the geometry of the molecule from the electrostatic repulsion between the bonding & non bonding pairs. Count the number of bonded electron pairs and the lone pairs to the central atom.
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VSEPR Theory The theory depicts the orientation of bonds and how molecules form. Orientations vary based on the number of non-bonded or lone pair electrons present in the molecule. Angles also change due to the number of lone pairs of electrons.
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Arrangement of electron pairs
VSEPR Model Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AX2 2 linear linear AX3 3 trigonal planar AX4 4 tetrahedral tetrahedral AX5 5 trigonal bipyramidal AX6 6 octahedral octahedral
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VSEPR Model With Lone Pairs
Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AX3 3 trigonal planar trigonal planar AX2E 2 1 trigonal planar bent AX4 4 tetrahedral tetrahedral AX3E 3 1 tetrahedral trigonal pyramidal AX2E2 2 2 tetrahedral bent
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VSEPR Model With Lone Pairs
Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry trigonal bipyramidal trigonal bipyramidal AX5 5 AX4E 4 1 trigonal bipyramidal distorted tetrahedron AX3E2 3 2 trigonal bipyramidal T-shaped trigonal bipyramidal AX2E3 2 3 linear
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VSEPR Model With Lone Pairs
Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB6 6 octahedral AB5E 5 1 octahedral square pyramidal AB4E2 4 2 octahedral square planar
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VSEPR Model With Lone Pairs
Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB6 6 octahedral AB5E 5 1 octahedral square pyramidal AB4E2 4 2 octahedral square planar
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Hybridization Hybridization is the mixing of two or more atomic orbitals to form a new set of hybrid orbitals. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shapes from the original. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process
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Predicting Hybridization of the Central Atom
Draw the Lewis Dot Structure of the molecule. Count the number of lone pairs and the number of atoms bonded to the central atom.
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Hybridization Table Number of Bonded Pairs and Lone Pairs
Examples 2 sp BeCl2 3 sp2 BF3 4 sp3 H2O, NH3 5 sp3d PCl5 6 sp3d2 SF6
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Hybridization
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Practice Problems Ex: Identify the hybridization of the following molecules: CS2 SO3 OF2 SF6 NH3
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Sigma () and Pi () Bonds
Sigma bonds () are formed by head-to-head overlap. Pi () bonds are formed by side-to-side overlap.
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Sigma and Pi Bonds Single Bond 1 sigma bond Double Bond 1 sigma and 1 pi bonds Triple Bond 1 sigma and 2 pi bonds Ex: How many sigma and pi bonds are in the CH3COOH molecule?
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σ and π Bonds Ethane (1 σ bond) Ethylene (1 σ bond and 1 π bond)
Acetylene(1 σ bond and 2 π bonds)
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Molecular Orbital Configuration
Molecular orbitals (MO): The number of MO formed is always equal to the number of atomic orbitals combined The filling of MO’s proceeds from low to high energies Each MO has a maximum of two electrons per orbital The number of electrons in the MO’s is equal to the sum of all of the electrons on the bonding atoms Electrons in the same orbital have opposite spin (Hund’s Rule)
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MO Theory Whenever two atomic orbitals overlap, two molecular orbitals are formed: one bonding, one antibonding: Bonding orbitals have lower energies and greater stability than atomic orbitals from which those are formed from. Antibonding orbitals have higher energy and lower stability than the atomic orbits from which they were formed. σ and σ* orbitals from s and p atomic orbitals. π and π* orbitals from p atomic orbitals.
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Molecular Orbital Summary of Second Row Diatomic Molecules
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Bond Order Ex: Write the molecular orbital electron configuration for the ion O2- and calculate the bond order of the ion.
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Paramagnetism and Diamagnetism
Paramagnetism – Unpaired electrons Diamagnetism – Paired electrons Ex: Write the molecular orbital electron configuration for the ion C2+ and identify whether the molecule is paramagnetic or diamagnetic.
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