Molecular geometry
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Molecular geometry

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Molecular geometry Molecular geometry Document Transcript

  • High School Chemistry Rapid Learning Series - 17 Rapid Learning Center Chemistry :: Biology :: Physics :: Math Rapid Learning Center Presents … p g Teach Yourself High School Chemistry in 24 Hours 1/32 http://www.RapidLearningCenter.com Molecular Geometry HS Ch i t R id Learning Series Chemistry Rapid L i S i Wayne Huang, PhD Kelly Deters, PhD Russell Dahl, PhD Elizabeth James, PhD Rapid Learning Center www.RapidLearningCenter.com/ © Rapid Learning Inc. All rights reserved. © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 1
  • High School Chemistry Rapid Learning Series - 17 Learning Objectives By completing this tutorial you will learn… Valence Shell Electron Pair Repulsion Theory p y Electronic and Molecular Geometry 3/32 Concept Map Previous content Chemistry New content Studies Matter One type is Valence Valance Bond Bond Theory Theory 1 bonding theory is Shown with Lewis Lewis Structures Structures Compounds Molecular Geometry Used to determine Electronic Geometry 4/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 2
  • High School Chemistry Rapid Learning Series - 17 Electronic and Molecular Geometry 5/32 VSEPR Theory V alence S hell E lectron i P air Repulsion Theory 6/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 3
  • High School Chemistry Rapid Learning Series - 17 What Does VSEPR Mean? Bonds are made of shared electrons (negatively charged subatomic particles). Negatively charged things repel each other. Valence Sh ll El t V l Shell Electron P i Pair Geometry: Bonds form as far apart from each other (and other electrons) as possible. To predict geometry Repulsion R l i VSEPR is a model used to predict molecular geometry based on minimizing the electrostatic repulsion of valence electrons around a central atom in a molecule. 7/32 Definition: Electronic & Molecular Geometry Electronic Geometry – Uses the VSEPR theory with the electron y regions around the central atom. An electron bond is a bond (single, double or triple…they all count as one region). Electron Geometry: H2O = Tetrahedral (with 2 electron lone pairs) Molecular Geometry – Uses the VSEPR theory with the atoms bonded around the central atom (spatial arrangement). 8/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com Molecular Geometry: H2O = Bent (No electrons shown) 4
  • High School Chemistry Rapid Learning Series - 17 How to Count Electron Regions Electronic geometry depends on electron regions surrounding the central atom. 1 NH3 has 4 electron regions. 2 H N H 3 H Each Bond Type Electron Region Single Bond 4 1 H C Double Bond N 1 1 Triple Bond 1 Lone Pair HCN has 2 electron regions. 1 2 The triple bond has only one electron region. 9/32 Chemical Formulas for Geometry Each geometry has a “generic” chemical formula. Central Atom Lone Pairs on Central Atom A X E Two Types of Electron Regions: E – “Lone Pair” Electron Region X – “Bonded” Electron Region Atoms Bonded to Central Atom (“Ligand”) e.g. AX3E X A X X 10/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 5
  • High School Chemistry Rapid Learning Series - 17 Linear Geometry AX2 - Central Atom - Electron Region 2 electron regions Bonds 180° apart Named: looks like a “line”. Examples: CO2, BeH2 11/32 Trigonal Planar Geometry AX3 - Central Atom - Electron Region 3 electron regions Bonds 120° apart Named: It’s a flat (“planar”) triangle. Examples: BF3, C2H4 12/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 6
  • High School Chemistry Rapid Learning Series - 17 Tetrahedron Geometry AX4 - Central Atom 4 electron regions - Electron Region Bonds 109.5° apart Named: If each plane (defined by 3 points) is covered, it’s a 4 (“tetra”) sided object (“hedron”). Examples: CH4, SO2Cl2 13/32 Trigonal Bipyramidal Geometry AX5 5 electron regions - Central Atom - Electron Region Bonds: Inside “Triangle” = 120° Triangle 120 Between top/triangle/bottom = 90° Named: If each plane (defined by 3 points) is covered, 2 pyramids with triangular bases are sitting base-to-base. Examples: PCl5, AsF5 14/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 7
  • High School Chemistry Rapid Learning Series - 17 Octahedron Geometry AX6 6 electron regions - Central Atom - Electron Region Bonds: 90 90° Named: If each plane (defined by 3 points) is covered, it’s an 8 (“Octa”) sided object (“hedron”). Examples: SF6, PF615/32 Determining Electronic Geometry Electronic geometry is determined by electron regions. Electron regions Angle between regions 2 Linear 180° 3 Trigonal Planar 120° 4 Tetrahedral 109.5° 5 Trigonal Bipyramidal 90° and 120° 6 16/32 Name of geometry Octahedron Picture 90° Electronic Geometry Mnemonic: Linear(2) – Trigonal Planar(3) – Tetrahedral (4) – Trigonal Bipyramidal(5) – Octahedron(6) = Long TriP To TriBe Overseas. © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 8
  • High School Chemistry Rapid Learning Series - 17 Determining Molecular Geometry Molecular geometry is determined by # of atoms bonded to the central atom (= #Electron Regions - #Lone Pairs). Formula Name of geometry Angle between bonded atoms AX2 Linear 180° AX3 Trigonal Planar 120° AX4 Tetrahedral 109.5 109 5° AX5 Trigonal Bipyramidal 90° and 120° AX6 Octahedron Picture 90° 17/32 Geometry with Lone Pairs Use the formula to determine the electronic geometry. Each atom bonded to the central atom (ligand) counts as 1. Each lone pair on the central atom counts as 1. e.g. AX2E2 2 bonds & 2 lone pairs = 4 electron regions Electronic: Tetrahedral Molecular: Bent To determine molecular geometry, first determine the electronic geometry and then remove lone pairs and re-name the geometry. 18/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 9
  • High School Chemistry Rapid Learning Series - 17 Bent Geometry with 1 Lone Pair AX2E E Remove 1 “E” Start with 3 electron regions (2X+E). Trigonal Planar Bent 120º Named: It looks like a “bent” line. Examples: SO2, O3 19/32 Bent Geometry with 2 Lone Pairs AX2E2 E Remove 2 “E”s E Start with 4 electron regions (2X+2E). Tetrahedron Bent 109.5º Named: It looks like a “bent” line. Examples: H2O, SF2 20/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 10
  • High School Chemistry Rapid Learning Series - 17 Trigonal Pyramidal Geometry AX3E E Remove 1 “E” Start with 4 electron regions (3X+E). Tetrahedron Trigonal Pyramidal Named: It looks like a pyramid with a triangular base. Examples: NH3, SOCl2 21/32 See-Saw Geometry AX4E E Remove 1 “E” Start with 5 electron regions (4X+E). Trigonal Bipyramidal Named: It looks like a see-saw. Examples: SF4, O2XeF2 See-Saw Note: This geometry has two different ligand positions, axial and equatorial. The lone pair at equatorial would be less crowded and more favorable. 22/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 11
  • High School Chemistry Rapid Learning Series - 17 T-Shaped Geometry AX3E2 E Remove 2 “E”s E Start with 5 electron regions (3X+2E). Trigonal Bipyramidal Named: It looks like a “T”. Examples: ClF3, BrF3 T-Shaped Note: This geometry has two different ligand positions, axial and equatorial. The lone pairs at equatorial would be less crowded and more favorable. 23/32 Linear with 3 Lone Pairs AX2E3 E E Remove 3 “E”s E Start with 5 electron regions (2X+3E). Trigonal Bipyramidal Named: It looks like a “line”. Examples: XeF2, I3- Linear Note: This geometry has two different ligand positions, axial and equatorial. The lone pairs at equatorial would be less crowded and more favorable. 24/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 12
  • High School Chemistry Rapid Learning Series - 17 Square Pyramidal Geometry AX5E E Remove 1 “E” Start with 6 electron regions (5X+E). Octahedron Square Pyramidal Named: It looks like a pyramid with a square base. Examples: BrF5, IF5 25/32 Square Planar Geometry AX4E2 E Remove 2 “E”s E Start with 6 electron regions (4X+2E). Octahedron Square Planar Named: It looks a flat (“planar”) square. Examples: ICl4-, XeCl4 26/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 13
  • High School Chemistry Rapid Learning Series - 17 Effect of Lone Pairs on Molecular Geometry Both molecules have 4 electron regions: H H C H H H N H H These electrons (lone pair) are not being “controlled” by another nucleus. Lone pairs take up more space than a bonding pair—they distort the bond angles slightly (push the bonding pairs closer). Instead of being 109.5° angles (tetrahedron), the angles are 107.3°. 107.3° 27/32 H N H H 107.3° 107.3° 2D Drawing 3D Drawing Determining Geometry with Lone Pairs Formula Name of geometry AX2E Bent AX2E2 Bent AX3E Trigonal Pyramidal AX4E See-Saw AX3E2 T-Shaped AX2E3 Linear AX5E Square pyramidal AX4E2 Picture Square planar 28/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 14
  • High School Chemistry Rapid Learning Series - 17 Determining Geometry - Example Example: Give the electronic and molecular geometry for H2O. Lone Pair 1 2 Bond H O H Bond 4 3 Lone Pair Step 1: Draw the Lewis Structure. Step 2: Determine # of electron regions (bonding and lone pair regions). Step 3: Determine electronic geometry (based on # of electron regions). 4 electron regions = tetrahedral Step 4: Determine molecular geometry. Count atoms bonded to and lone pairs around the central atom. Ignore the lone pairs and adjust the geometry (bond angles). H2O: 2 bonded atoms and 2 lone pairs = bent molecular geometry. 29/32 Note: This problem is commonly answered incorrectly as it “looks” linear as it’s written above. However the lone pairs make it bent! Learning Summary Valence Shell Electron Pair Repulsion Theory (VSEPR) is used to determine geometry of molecules (by minimizing the electroni i i i th l t pair repulsion). Electronic geometry is determined by electron regions, while molecular geometry is determined by atoms bonded t th b d d to the central t l atom (bonding pairs). 30/32 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 15
  • High School Chemistry Rapid Learning Series - 17 Congratulations You have successfully completed the tutorial Molecular Geometry Rapid Learning Center Rapid Learning Center Chemistry :: Biology :: Physics :: Math What’s N t Wh t’ Next … Step 1: Concepts – Core Tutorial (Just Completed) Step 2: Practice – Interactive Problem Drill Step 3: Recap – Super Review Cheat Sheet Go for it! 32/32 http://www.RapidLearningCenter.com © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 16