MOLECULAR GEOMETRY

1. MOLECULAR GEOMETRY

2. Recap • Bonding types: • Ionic • Transfer of e- • Attraction between ions • Covalent (molecular) • Sharing of e- • Covalent network • Metallic

3. Objectives • Be able to determine the Lewis Dot structure for a molecule • Know what a coordinate covalent bond is • Be able to identify a coordinate covalent bond within a molecule • Identify elements in a molecule that might violate the octet rule • Know what VSEPR is • Use VSEPR to explain geometry • Identify electron pair geometry • Identify molecular geometry

4. How do they share • Satisfied octet • Octet rule: full s & p sub shell (usually 8 electrons but not always) is “stable” • Lewis dot structures • Shows how elements bond in covalent compounds while satisfying their octet

5. Two ways to “Lewis Dot” • Atoms with Valence e- • Like a puzzle, assembled piece by piece • Pro: great for small molecules because it’s fast • Con: hard for large molecules or molecules with complex bonding • “Valence e- pool” method • Strip valence e- from atoms in compound, assemble compound and add valence e- back in as you go • Pro: Great for complex molecules • Con: Doesn’t explain why elements bond the way they do

6. Atoms w/ valence e- • NCl3 • SO • SO2 • Coordinate covalent bond- one atom donates both electrons in the bond • This method makes it easier to see these • What to look for: • The number of e- not involved in bonding is = to its total valence e-

7. Rules for “Valence e- pool” method • Determine the number of valence electrons in compound • if molecule has a charge determine how many e- it lost or gained and include this in the total # of valence e- Ex: SiS2 Si has 4 & S has 6 and there are two of them for a total of 16 valence e- Try these: FCl NH3 CH3OH SO2

8. Rules continued • Determine the central atom • The central atom is usually the atom that needs the MOST valence e- to be stable • doesn’t matter if only 2 atoms • if all atoms need the same # choose the atom with lower electronegativity (typically the atom that is first) Ex: SiS2 Si needs 4 S only needs 2, Si is central atom Try these: FCl NH3 CH3OH SO2

9. Rules continued • Write the symbol of central atom with the symbols of other elements around it. Then connect atoms with single bonds (subtract bonding e- from the total valence e-) Ex: S Si S Total v e- 16 bonding e- 4 remaining e- 12 Try these: FCl NH3 CH3OH SO2

10. Rules continued 4. Place remaining valence e- as lone pairs. • Most electroneg. atom first, until satisfied • 2nd most electroneg. next, etc. • check total # of valence e- to make sure you used them all (S is most electronegative here) Ex: S Si S 12-12 = 0 v e- left Good! Try these: FCl NH3 CH3OH SO2

11. Rules continued • Check each atom to see if it is satisfied. • Octet rule • If there’re atoms not satisfied, move lone pairs to form double or triple bonds. • If charged put it in brackets & put a charge on it Ex: S Si S Here we see that Si is not satisfied but we CANNOT add any more e-!! Move a lone pair from each S to make a bond. S = Si = S Try these: FCl NH3 CH3OH SO2

12. Octet rule violators • Some elements can violate the octet rule • Low side: these elements don’t need to have full s & p sub-shells • Boron most notable, can sometimes have just 6 (BH3) • High side: These elements can have as many as 12 valence e- • Any element in the 3rd period or below

13. Lewis Dot Practice • Complete the first column of the worksheet

14. MOLECULAR GEOMETRY Molecule adopts the shape that minimizes the electron pair repulsions. VSEPR • Valence Shell Electron Pair Repulsion theory. • Most important factor in determining geometry is relative repulsion between electron pairs.

15. Electron pairs (densities) • VSEPR is all about the repulsion of electrons • Focus is on areas of electron density instead of individual electrons • Lone pair = one area • Single bond = one area • Double bond = one area • Triple bond = one area Basically everything is only 1 area

16. Gumdrop molecules • Per group of two people • 7 gumdrops (atoms) • 6 toothpicks (electron densities) • CO2 • CH4 • NH3 • H2O • SO3

17. Electron Pair GeometriesFigure 9.12

18. What is meant by region of electron density? • Could be a BONDED pair of electrons • Could be a LONE pair of electrons

19. Electron Pair Geometry • Does NOT distinguish between atoms (bonding pairs) and lone pairs • Molecules will be one of 5 electron geometries • Draw the shape of the molecular geometry and write the Electron Pair Geometry in the square

20. 2 regions180° bond angle

21. 3 regions

22. o o

23. 4 regions

24. o o o

25. Electron Pair GeometriesFigure 9.12

26. •• H H N H Structure Determination by VSEPR Ammonia, NH3 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron.

27. Structure Determination by VSEPR Ammonia, NH3 There are 4 electron pairs at the corners of a tetrahedron. The ELECTRON PAIR GEOMETRY is tetrahedral.

28. Structure Determination by VSEPR Ammonia, NH3 The electron pair geometry is tetrahedral. The MOLECULAR GEOMETRY — the positions of the atoms — is TRIGONAL PYRAMIDAL.

29. Structure Determination by VSEPR Water, H2O 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. The electron pair geometry is TETRAHEDRAL.

30. Structure Determination by VSEPR Water, H2O The electron pair geometry is TETRAHEDRAL The molecular geometry is BENT.

31. Final Column • Molecular Geometry

32. Geometries for Four Electron PairsWhy are the angles different?How do the repuslive forces of lone pairs differ from bonded pairs?

33. O • • • • H C H O • • • • C H H Structure Determination by VSEPR Formaldehyde, CH2O 1. Draw electron dot structure 2. Count BP’s and LP’s at C 3. There are 3 areas of electron density around C at the corners of a planar triangle. The electron pair geometry is PLANAR TRIGONAL with 120o bond angles.

34. Structure Determination by VSEPR Formaldehyde, CH2O The electron pair geometry is TRIGONAL planar The molecular geometry is also trigonal planar.

35. Phenylalanine, an amino acid

36. Phenylalanine

37. Structures with Central Atoms with More Than or Less Than 4 Electron Pairs Often occurs with Group 3A elements and with those of 3rd period and higher.

38. Boron Compounds Consider boron trifluoride, BF3 The B atom is surrounded by only 3 electron pairs. Bond angles are 120o Geometry described as planar trigonal

39. 5 regions

40. Compounds with 5 Pairs Around the Central Atom o o 5 electron pairs

41. Compounds with 8 pairs around the central atom

42. Sulfur Tetrafluoride, SF4 • Number of valence electrons = 34 • Central atom = S • Dot structure o Electron pair geometry --> trigonal bipyramid(because there are 5 pairs around the S) o

43. Sulfur Tetrafluoride, SF4 Lone pair is in the equator because it requires more room. o o

44. Molecular Geometries for Five Electron PairsFigure 9.14

45. Compounds with 6 Pairs Around the Central Atom o o 6 electron pairs

46. Molecular Geometries for Six Electron Pairs