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Chapter Ten Chemical Bonding ll Molecular Geometry and Hybridization of Atomic Orbitals
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Molecular Geometry
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The Valence-Shell Electron-Pair Repulsion (VSEPR) Method based on the idea that pairs of valence electrons in bonded atoms repel one another. Assumes electron pairs try to get as far apart as possible Each electron pair or bond takes up ~ same amount of space # of bonds or pairs determines molecular geometry Molecular Geometry: The shape of a molecule that describes the location of nuclei & the connections between them.
Molecules with No Lone Pairs Bond angles due to # of repulsions Each bond takes up space of 1 electron pair AB2
AB3
Linear
Trigonal planar
AB5
Trigonal bipyramid
AB4
Tetrahedral
AB6
Octahedral
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Molecules with Lone Pairs: Know table 10.2 Lone pair electrons not seen but take up space Act as “invisible bond” Count electrons as E’s Single, double or triple bonds count as 1 bond
To determine molecular geometry
Add up all the B’s and E’s on the molecule
H O H B A B AB2E2 The sum equals number of spaces needed 2B + 2E = 4 # spaces = 4 Match to table of geometries without lone pairs
Electron pair geometry: Tetrahedral Molecular Geometry: Bent
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Molecules with More than 1 Central Atom
VSEPR must be done separately for each atom
May result in a different molecular geometry around each one
Methanol CH3OH C: 4 spaces: tetrahedral O: 4 spaces: bent H H
C
O
H
H
Oxoacids: Hydrogen goes on oxygens
HNO3, H2SO4, etc. will also use this method
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Dipole Moments
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Dipole Moments and Polar Molecules electron poor region
electron rich region
H
F
= Q x r
Q is the charge r is the distance between charges 1 D = 3.36 x 10-30 C m
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Predicting Polarity: CO2
Predict molecular shape VSEPR Linear
AB2 O=C=O
Predict bond dipoles C less electronegative than O
C-O
Bond dipoles cancel or combine? Cancel
Nonpolar
O=C=O
ď =0
Predicting Polarity: NH3
Predict molecular shape. VSEPR Tetrahedral
AB3E
Predict bond dipoles. H less electronegative than N N-H lone pair more electronegative than N
Bond dipoles cancel or combine? Combine: Polar molecule ď >0
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Polarity of different Molecules Isomers:
Same molecular formula Different structure
Cis Large groups on same side of double bond plane
Trans Large groups across plane of double bond
Dichloroethylene: C2H2Cl2 2 possible isomers
Cis-dichloroethylene
Trans-dichloroethylene
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Valence Bond Theory and Hybridization
Valance Bond Theory: Formation of H2(g) Covalent bond formation Electrons in 1s atomic orbitals pair Opposing spins occupy the overlap region between 2 atoms Shield nuclei from each other
Delocalization Area of high electron density (red) Lowers energy, provides stability Bonding electrons are found in the overlap region (covalent bond)
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Carbon Bonding Electronic configuration C should have 2 bonds 2 half-filled orbitals on C
[He] 2s22px12py12pz0
Experimentally C has 4 identical bonds: CH4 Implies 4 half-filled orbitals [He] 2s12px12py12pz1 Excite 2s1 electron to 2pz orbital
Problems with Theory 4 bonds, but orbitals of differing energies, bond lengths 3 bonds: H 1s C 2p Higher energy 1 bond: H 1s C 2s Lower energy
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Hybrid Orbitals Model that predicts shape based on atomic orbitals Allows use of electrons in s, p and d orbitals in bond Creates several identical bonds “Averages� orbital energies to equalize bonds
Used for central atoms in covalent bonds
# hybrid orbitals = # combining atomic orbitals
Use VSEPR and Lewis theory to predict geometry
Determine Lewis structure and VSEPR notation Orientation of determines electron geometry Determine hybridization based on VSEPR model Electron pairs may occupy 1 or more of the hybrid orbitals
sp3 Hybridization 4 equivalent orbitals
1part s to 3 parts p: sp3
CH4 : 4 valence electrons NH3 : 5 valence electrons
Orbitals point toward corners of tetrahedron
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sp2 Hybridization
3 sp2 hybrid orbitals in a plane
1 2s orbital & 2 2p orbitals Forms 3 sp2 hybrids with 1 empty 2p orbital Trigonal planar geometry: 120o angles. Often involves double bonds
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sp Hybridization Two sp orbitals in a plane
1 2s orbital & 1 2p orbitals Forms 2 sp hybrids and 1 empty 2p orbital Linear geometry: 180o angles Triple bonds may be present
BE: 2 2s valence electrons 2 sp hybrid orbitals
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Hybrid Orbitals Involving d Subshells Allows for expanded valence shell compounds 3s electron promoted to 3d subshell Five sp3d hybrid orbitals. Trigonal bipyramidal molecular geometry
3s & 3p electrons promoted to 3d subshell Six sp3d2 hybrid orbitals Octahedral molecular geometry.
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Hybrid Orbitals: d and f Subshells Allows for expanded valence shell compounds
A 3s and a 3p electron are promoted to 3d subshell Makes 6 sp3d2 hybrid orbitals
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Bonding Scheme for Iodine Pentafluoride (IF5) VSEPR AX5E Electron Geometry Octahedron Molecular Geometry Tetragonal Pyramid Bonding 5 sp3d2 I - F bonds 1 electron pair in sp3d2 orbital
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Hybrid Orbitals and Geometric Orientations
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Hybridization of Double and Triple Bonds
Carbon Bonding: sp2 Hybridization of CH2O
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10.5
Sigma and Pi Bonding in Ethylene Ethylene Sigma Bonding (ď ł) End to end s,p (or d) orbitals Single bonds Pi Bonding (ď °) Parallel side to side
C: 3 sp2 orbitals H: 1s orbital C: 1 p orbital Double bond 1e- from each C
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Bonding in Acetylene, C2H2 sp Hybridization
Sigma Bonding End to End C: 2 sp orbitals H: 1s orbital
HC CH
Pi Bonding Side to Side 2 p orbitals per C
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Molecular Orbital Theory
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Molecular Orbitals and Bonding In H2 (1s orbitals)
Molecular orbitals (MOs)
Orbitals that result from the interaction of atomic orbitals Interaction can stabilize or destabilize molecule
Higher energy than atomic orbitals No electron density in center.
Bonding Orbital
Lower energy than atomic orbitals: High charge density in center
Antibonding Orbital
Higher in energy, designated with a *
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Molecular Orbitals From 2p Atomic Orbitals 6 atomic orbitals
6 molecular orbitals
Bond Order: Can the Molecule Exist? BO = ½ (# bonding electrons - # antibonding electrons)
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