Chapter 6 –The Shape of Molecules

1. Chapter 6 –The Shape of Molecules

2. Why Care about Shapes of Molecules • Dot diagrams show how atoms are connected in a molecule and how valence electrons are arranged • Problem: not 3D

3. Shape determines molecular properties Melting point Boiling point Chemical reactivity Why Care About the Shape of Molecules

4. Theory that determines how molecules arrive at a three dimensional shape Repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible All electrons are negatively charged. Like charges repel. VSEPR Model (Valence Shell Electron Pair Repulsion)

5. Are These Bonds As Far Apart As Possible? No Way! H H H C H H B H H

6. Draw Lewis structure. Determine the number of electron groups around the central atom 1 electron group is equal to Single, Double, Triple Bond or Lone Pair Count up lone pairs on the central atom Determine the shape of the molecule Determine the bond angle Predicting a VSEPR Structure

7. 2 Electron Groups Linear Bond angles of 180˚ X A X Base VSEPR Structure

8. 3 Electron Groups Trigonal Planar Bond angles of 120˚ X A X X Base VSEPR Structure

9. 4 Electron Groups Tetrahedral Geometry Bond angles of 109.5˚ X X A X X Base VSEPR Structure

10. Determine shape and bond angles for BI3 SiBr4 BeF2 Your Turn Trigonal Planar, 120° Tetrahedral, 109.5° Linear, 180°

11. Lone Pairs of Electrons make the bond angle a little bit smaller More repulsion Lone Pairs of Electrons

12. Bent – Bond Angles of < 120 1 Lone Pairs on Trigonal Planar A X X

13. Trigonal Pyramidal – Bond Angles 107 X A X X 1 Lone Pair w/ Tetrahedral

14. Bent – Bond Angles = 104.5 2 Lone Pairs w/ Tetrahedral A X X

15. Determine shape and bond angles for PBr3 O3 SeO2 Your Turn Trigonal Pyrimidal, 107° Bent, <120° Bent, <120°

16. The ability of an atom in a molecule to attract shared electrons to itself. Developed by Linus Pauling Values range 0.7 to 4.0 Fluorine = 4.0 Francium and Cesium = 0.7 What is the periodic trend? Top to Bottom – Decrease Left to Right Increase Electronegativity

17. A polar molecule is one that has a partially positive and partially negative side Molecules are Always nonpolar if they are one of the 3 base shapes w/ the same atom at the ends Molecules are Always polar if their bond dipoles do not cancel out Molecules are polar if they do not have the same atoms at the end, or bent, or pyramidal Molecular Polarity

18. Forces of attraction between molecules Intermolecular Forces are responsible for melting and boiling points Strong IMF raise BP and MP Three different types of IMF London Forces, Dipole-Dipole, and Hydrogen Bonding Intermolecular Forces

19. Permanent attraction between polar molecules Dipole - Dipole

20. Extra Strong attraction between polar molecules where H is bonded to F,N,O Hydrogen Bonding

22. Temporary Dipole caused by the collision of nonpolar molecules London Forces

24. Arrange the following in order of increasing boiling points H2, NH3, PH3, H2O H2 = -235 ºC PH3 = -88 ºC NH3 = -33 ºC H2O = 100 ºC So What?