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CHEMISTRY 161 Chapter 10 Chemical Bonding II
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PREDICTING THE GEOMETRY OF MOLECULES
1. derive Lewis structure of the molecule 2. discriminate between bonding and non-bonding electron pairs O H 3. VALENCE SHELL ELECTRON PAIR REPULSION
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2. electrons repel each other
VALENCE SHELL ELECTRON PAIR REPULSION VSEPR 1. identify in a compound the central atom 2. electrons repel each other 3. valence electron pairs stay as far apart as possible 4. non-bonding electrons repel more than bonding electrons
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central atom no non-bonding pairs non-bonding pairs
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TWO ELECTRON PAIRS AROUND BERYLLIUM ATOM
AB2 BeCl2 Cl Be Cl TWO ELECTRON PAIRS AROUND BERYLLIUM ATOM
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LINEAR ARRANGEMENT BEST
Cl Be Be 180° Be 90° 270° LINEAR ARRANGEMENT BEST IT PUTS ELECTRON PAIRS FURTHEST APART
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THREE ELECTRON PAIRS AROUND THE BORON ATOM
AB3 BF3 F F B F THREE ELECTRON PAIRS AROUND THE BORON ATOM
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THREE ELECTRON PAIRS AROUND THE BORON ATOM
F B THREE ELECTRON PAIRS AROUND THE BORON ATOM TRIGONAL PLANAR ARRANGEMENT BEST B 120°
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MOLECULAR SHAPE F B F F THE SHAPE OF BF3 IS TRIGONAL PLANAR.
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AB4 CH4 H C four electron pairs expect square planar 90° C
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better arrangement for four electron pairs
TETRAHEDRAL C 109.5° bigger than 90 ° in square planar tetrahedral 4 electron pairs put on the H-atoms
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TETRAHEDRAL H C C 109.5° H H H shape of CH4 is tetrahedral
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AB5 PF5 FIVE ELECTRON PAIRS AROUND PHOSPHORUS P F P 5 electron pairs
trigonal bipyramidal
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shape of PF5 is trigonal bipyramidal
Bond angle F 900 F P F P F 1200 F shape of PF5 is trigonal bipyramidal two of the F atoms different from the others
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AXIAL Bond angle F 900 F F P F 1200 EQUATORIAL F
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AB6 six electron pairs around the sulfur atom SF6 F F F S S F F F
octahedral
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900 F F F S S F F F shape of SF6 is octahedral
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central atom no non-bonding pairs non-bonding pairs
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AB2E AB3 SeO2 O Se O
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VSEPR treats double bonds like a single bond O Se
THREE ELECTRON PAIRS AROUND SELENIUM ELECTRON PAIR GEOMETRY Se TRIGONAL PLANAR
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THE MOLECULAR SHAPE IS THE POSITION OF THE ATOMS
Se O Se ADD OXYGENS SeO2 IS V-SHAPED (OR BENT) THE MOLECULAR SHAPE IS THE POSITION OF THE ATOMS
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electron pairs around the nitrogen atom
AB3E AB4 H N H NH3 H electron pairs around the nitrogen atom
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NH3 is trigonal pyramidal
PUT ON THE 3 H ATOMS N H H H NH3 is trigonal pyramidal
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four electron pairs around the oxygen atom
AB2E2 AB4 H O H four electron pairs around the oxygen atom PUT ON THE 2 H-ATOMS O O H H shape of H2O is V-shaped or bent
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AB4E AB5 SF4 S F F F S F TRIGONAL BIPYRAMID
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WHERE DOES LONE PAIR GO? OR
F S F OR lone pairs occupy the trigonal plane (the “equator”) to minimize the number of 90° repulsions
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AB4E AB3E2 AB2E3 SF4 1 lone pair See-saw shaped ClF3 2 lone pairs T-shaped XeF2 3 lone pairs Linear F F F F Xe S Cl F F F F F F lone pairs occupy the trigonal plane (the “equator”) first to minimize the number of 90° repulsions
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AB5E AB6 BrF5 Br F Br Square pyramidal
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AB6 AB4E2 XeF4 Xe F : Xe Xe F : Xe F lone pairs MUST BE AT 1800
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Total valence electron pairs Electron Pair Geometry
Summary of Molecular Shapes Total valence electron pairs Electron Pair Geometry Lone electron pairs Shape of Molecule 2 Linear Linear Trigonal planar Trigonal planar 3 1 V-shaped Tetrahedral 4 Tetrahedral 1 Trigonal pyramid 2 V-shaped
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Total valence electron pairs Electron Pair Geometry
Lone electron pairs Shape of Molecule Trig. bipyramid. 1 See-saw Trigonal bipyramidal 5 2 T-shaped 3 Linear Octahedral 6 Octahedral 1 Square pyramid 2 Square planar
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POLYATOMICS molecules with no single central atom we apply our VSEPR rules to each atom in the chain Example: ETHANOL
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ETHANOL C2H5OH O C H The atoms around the carbons form a.
tetrahedral arrangement The atoms around the oxygen form a V-shaped structure.
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H C O
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EXAMPLES BF4- ICl4- Cl2O SO2Cl2 Cl2CO Cl2SO N2F2 NH4+ NH2OH
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Homework Chapter 10 pages , problem sets
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