Shapes of Molecules David Read. Key Aims Revise A-level VSEPR theory. Prepare for Phil Gale’s lectures on symmetry, isomers, NMR etc.

23.2.7 Tetrahedral vs. Square Planar Geometry Example
23.2.7 Tetrahedral vs. Square Planar Geometry Example

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Shapes of Molecules David Read

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Key Aims Revise A-level VSEPR theory. Prepare for Phil Gale’s lectures on symmetry, isomers, NMR etc.

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Warm-up: How many different molecular shapes can you think of? Give an example for each shape and sketch the structure.

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How many shapes did you identify? A.4 B.5 C.6 D.7 E.8 F.9 G.10 H.>10

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Underlying principle (see Shriver & Atkins) In the VSEPR model, regions of enhanced electron density take up positions as far apart as possible. In the VSEPR model, electron pairs take up positions as far apart as possible. EPs can be bonding pairs (BPs) or lone pairs (LPs).

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How confident do you feel about using VSEPR to determine shapes? 1.It’s a doddle! 2.OK, but need to revise it a bit. 3.Not too confident. 4.I don’t remember much. 5.I don’t remember this at all!

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Step-by-step 1.Write a Lewis dot structure for the molecule. * A Lewis dot structure is like a dot-and-cross * diagram that only uses dots. e.g. methane C – 4 valence electrons H – 1 valence electron All 4 valence electrons on C pair up with an electron from an H atom, forming 4 bonds.

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Step-by-step 2.Count the number of atoms and lone pairs around the central atom. e.g. 4 atoms around carbon, and no lone pairs. 4 regions of electron density to distribute.

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Step-by-step 3.Work out how the regions of high electron density will be arranged around the central atom: 2 Regions – 3 Regions – 4 Regions – 5 Regions – 6 Regions – Linear Trigonal planar Tetrahedral Trigonal bipyramidal Octahedral

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Step-by-step 4.Draw the structure and label bond angles. 109.5º Tetrahedral

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Step-by-step 4.Draw the structure and label bond angles. 109.5º Tetrahedral

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Try one yourself… 5 What is the shape of a PF 5 molecule? Lewis structure Count e – pairs around central atom How many lone pairs are there? Draw structure and label bond angles. Press any button on your handset when you have completed the task. 0

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PF 5 The 5 bonding pairs will take up a trigonal bipyramidal arrangement:

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PF 5 The 5 bonding pairs will take up a trigonal bipyramidal arrangement:

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What is the shape of BeH 2 ? 1.Tetrahedral 2.Trigonal pyramidal 3.Linear 4.V-shaped 5.Octahedral

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Beryllium hydride Be – 2 valence electrons 2 bonds formed to H atoms Linear molecule 180º

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What is the shape of BF 3 ? 1.Tetrahedral 2.Trigonal planar 3.Linear 4.V-shaped 5.Octahedral

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Boron trifluoride B – 3 valence electrons 3 bonds formed to F atoms Trigonal planar molecule 120º

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What is the shape of SF 6 ? 1.Tetrahedral 2.Trigonal pyramidal 3.Linear 4.V-shaped 5.Octahedral

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Sulphur hexafluoride S – 6 valence electrons 6 bonds formed to F atoms Octahedral molecule 90º

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What about lone pairs?

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Which statement is correct? 1.Repulsion due to bonding pairs is greater than repulsion due to lone pairs. 2.Repulsion due to bonding pairs is the same as repulsion due to lone pairs. 3.Repulsion due to lone pairs is greater than repulsion due to bonding pairs.

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Step-by-step 1.Write a Lewis structure for the molecule. e.g. water O – 6 valence electrons H – 1 valence electron 2 of oxygen’s 6 valence electrons pair up with an electron from an H atom, forming 2 bonds. The remaining valence electrons make up 2 LPs.

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Step-by-step 2.Count the number of bonds and lone pairs around the central atom. e.g. Water 2 bonds and 2 LPs 4 regions of electron density to distribute

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Step-by-step 3.Work out how the electron regions will be arranged around the central atom: 2 Regions – 3 Regions – 4 Regions – 5 Regions – 6 Regions – Linear Trigonal planar Tetrahedral Trigonal bipyramidal Octahedral

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Step-by-step 4.Deduce the shape of the molecule based on the arrangement of atoms (not including LPs)

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Step-by-step 5.Draw the structure. What about the bond angle?

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Methane, ammonia and water 4 bonds, 0 lone pairs3 bonds, 1 lone pair2 bonds, 2 lone pairs TetrahedralTrigonal pyramidalBent (v-shaped) Bond angle = 109.5º This is the ideal tetrahedral angle

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What’s the bond angle in water? A.109.5º B.108º C.104.5º

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What’s the bond angle in ammonia? A.109.5º B.108º C.104.5º

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Methane, ammonia and water 4 bonds, 0 lone pairs3 bonds, 1 lone pair2 bonds, 2 lone pairs TetrahedralTrigonal pyramidalBent (v-shaped) Bond angle = 109.5ºBond angle = 108ºBond angle = 104.5º One lone pair reduces the bond angle slightly Two lone pairs reduce the bond angle more

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Try another one yourself… 6 What is the shape of a XeF 4 molecule? Lewis structure Count e – regions around central atom How many lone pairs are there? Draw structure and label bond angles. Press any button on your handset when you have completed the task. 2

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Xenon tetrafluoride Xe – 8 valence electrons 4 bonds formed to F atoms 4 electrons left = 2 LPs 6 regions octahedral 4 regions are bonds Square planar molecule

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Xenon tetrafluoride Xe – 8 valence electrons 4 bonds formed to F atoms 4 electrons left = 2 LPs 6 regions octahedral 4 regions are bonds Square planar molecule Bond angle = 90º

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What about double bonds?

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Carbon dioxide C – 4 valence electrons 2 double bonds formed to O atoms (all valence electrons used in bonds) 0 lone pairs 2 regions (both are bonds) Linear molecule

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Carbon dioxide C – 4 valence electrons 2 double bonds formed to O atoms (all valence electrons used in bonds) 0 lone pairs 2 regions (both are bonds) Linear molecule

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Try another one yourself… 3 What is the shape of a SO 2 molecule? Lewis structure Count e – regions around central atom How many lone pairs are there? Draw structure and label bond angles. Press any button on your handset when you have completed the task. 1

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What is the shape of SO 2 ? 1.Tetrahedral 2.Trigonal pyramidal 3.Linear 4.V-shaped 5.Octahedral

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Sulphur dioxide S – 6 valence electrons 2 double bonds formed to O atoms (4 valence electrons used in bonds) 2 electrons left 1 lone pair 3 regions trigonal planar 2 bonds to atoms v-shaped Linear molecule

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Challenge… What are the shapes of IF 7 and IF 5 molecules?

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Challenge… What are the shapes of IF 7 and IF 5 molecules? Pentagonal bipyramidal

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Challenge… What are the shapes of IF 7 and IF 5 molecules? Pentagonal bipyramidalSquare pyramidal

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How confident do you feel about using VSEPR to determine shapes? 1.It’s a doddle! 2.OK, but need to revise it a bit. 3.Not too confident. 4.I don’t remember much. 5.I don’t remember this at all!

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How useful did you find the zapper session? 1.Very Useful 2.Quite useful 3.No view either way 4.Not very useful 5.Waste of time

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