Structure of molecules

1. Structure of molecules • Objectives: • To understand molecular structure and bond angles • To learn to predictmolecular geometry from the number of electron pairs • To learn to apply the VSEPR model to molecules with double bonds

2. 3-D Molecular Structure • Lewis structures = arrangement of valence e- • Also want to talk about the 3-D geometric structure of molecules • Example: H2O • “Bent” shape with 104.5° angle between the two hydrogens

3. 3-D Molecular Structure • Linear structure • All atoms in a line • 180° angle between the oxygen atoms

4. 3-D Molecular Structure • Trigonal planar • Example: BF3 • Flat (planar) with 120° angles between fluorine atoms

5. 3-D Molecular Structure • Tetrahedral • Example: CH4 • Tetrahedron: A four-sided pyramid • 109.5° angles between Hs in methane

6. 3-D VSEPR Model • Molecular shape determines molecular properties!!!!!!! • Valence Shell Electron Pair Repulsion (VSEPR) Model allows us to make predictions about molecular shape • Pairs of electrons surrounding an atom repel each other • Atoms in molecule are positioned to minimize repulsion (stable state) • Bonding e-s and lone pairs are positioned as far apart as possible

7. VSEPR: 2 pairs of electrons • BeCl2 • First, Lewis diagram: • Be (2 valence e-) + 2 Chlorine (7 valence e-) = 16 valence e- • Beryllium is exception to octet rule - often forms “electron deficient molecules” • Cl - Be - Cl • Arrange remaining e- (16-4 = 12 e-) around the Chlorines

8. VSEPR: 2 pairs of electrons • BeCl2 has two pairs of electrons surrounding it • What is best way to arrange two pairs of electrons to minimize repulsions? • Put electron pairs 180° degrees apart • Now we can specify 3-D molecular structure: linear • Whenever you have 2 pairs of electrons surrounding an atom, they should always be placed at 180°

9. VSEPR: 3 pairs of electrons • BF3 - Boron trifluoride • Doesn’t follow octet rule • Boron tends to form “electron deficient” configurations • Boron (3 valence e-) + 3 Fluorine (7 valence e-) = 24 valence e- • Boron has 3 e- pairs around it • Distribute remaining e-(24-6= 18 e-) around the fluorines

10. VSEPR: 3 pairs of electrons • BF3 - Boron trifluoride • How to minimize repulsion between 3 pairs of electrons? • 120° angles between electron pairs • Trigonal planar structure • Whenever 3 pairs of e-around atom, should always be placed at corners of triangle, with 120° angle between...

11. VSEPR: 4 pairs of electrons • CH4 - Methane • Lewis Diagram • Carbon (4 valence e-) + 4 Hydrogen (1 valence e-) = 8 valence e- • Follows duet (hydrogen) and octet (carbon) rules

12. VSEPR: 4 pairs of electrons • CH4 - Methane • How to minimize repulsion between 4 pairs of electrons? • 109.5° angles between electron pairs • Tetrahedral shape (like a square pyramid) • Whenever 4 pairs of e-around atom, should always be placed at 4 corners of tetrahedron, with 109.5° angle between... • Whenever you have 4 pairs of e- around atom: tetrahedral geometry

13. Try it with Ammonia • NH3 - Ammonia • Draw Lewis structure • H - N - H | H • Nitrogen (5 valence e-) + 3 Hydrogen (1 valence e-) = 8 valence e- (6 already accounted for by lines) • Follow octet and duetrules • . .H - N - H | H

14. Try it with Ammonia • . .H - N - H | H • How many electron pairs? • 4 • How to reduce repulsions? • Tetrahedral geometry • Angles? • 109.5° - but angle between hydrogens actually smaller - lone pair more repulsive than bonded pair • We call this TRIGONAL PYRAMIDAL

15. Steps for predicting molecular geometry with VSEPR • Draw Lewis structure for molecule • Count electron pairs and arrange them in a way that minimizes repulsions: put them as far apart as possible • Determine positions of atoms from the way the electron pairs are shared • Determine name of molecular structure (linear, trigonal planar, tetrahedral, bent, trigonal pyramid)

16. VSEPR Model of Water • First - Lewis structure • H - O - H • Oxygen (6 valence e-) + 2 Hydrogen (2 valence e-) = 8 valence e- (4 accounted for by lines) • Follow octet and duet rules • ..H - O - H ··

17. VSEPR Model of Water • ..H - O - H ·· • How many electron pairs? • 4 • How to reduce repulsions? • tetrahedral geometry • Angles? • 109.5° angles (actually less, since lone pairs repulse more than bonded pairs) • We call this BENT shape

18. VSEPR and Molecules with Double Bonds • Carbon Dioxide • 2 electron pairs: linear structure (180° bond angle) • We know that there are 4 e- pairs around carbon, but not tetrahedral • 2 pairs of electrons between Carbon and each Oxygen • Each double bond should be treated as single bond for geometry purposes

19. Hybrid Orbitals • If ans and aporbital overlap, what happens????? • You get a linear sp orbital - a hybrid combination of the s and p orbitals • This is the type of hybridization you get with linear molecular geometry!

20. Hybrid Orbitals • If ans and 2 porbitals overlap, what happens????? • You get a trigonal sp2 orbital - a hybrid combination of the s and 2p orbitals • This is the type of hybridization you get with trigonal molecular geometry!

21. Hybrid Orbitals • If ans and 3 porbitals overlap, what happens????? • You get a tetrahedral sp3 orbital - a hybrid combination of the s and 3p orbitals • This is the type of hybridization you get with tetrahedral molecular geometry!

22. Guide to hybrid orbitals:

23. Questions • What is hybridization of CH4? • Procedure - count regions of electron density around the atom and match this number with the corresponding hybridization