Molecular Shape

1. Molecular Shape

2. Things to remember • The shape of a molecule: • determined by where the nuclei are located • nuclei go to certain locations because of its electron pairs

3. Use the Lewis Structure • Lewis structure is 2-D, but it can help you figure out the 3-D shape

4. Atom Lewis Structure # Unpaired Electrons Bonding Capacity . H H 1 1 .. F, Cl, Br, I ·F: 1 1 .. . C, Si ·C· 4 4 . .. N, P ·N· 3 3 . O, S ·O: 2 2 · ·· Ne, Ar, Kr :Ne: 0 0 ·· Bonding Capacity ..

5. Molecular Shape • Determined by overlap of orbitals • Shape determined by two factors: • The total number of atoms & • The number of electron pairsin different locationson central atom Classify electron pairs as bonding or nonbonding

6. Molecular Shape & VSEPR • Electron pairs repel each other: • want to be as far apart from each other as can be • Nonbonding pairs take up a little more room than bonding pairs

7. Total Number of Electron Pairs Orientation 1 or 2 Linear: 2 electron pairs can be 180 apart 3 Planar Triangular: 3 electron pairs - 120 apart 4 Tetrahedral: 3-D, 4 electron pairs - 109 apart Orientation of Electron Pairs

8. 2-Atom Molecules • Atoms are located right next to each other • these are linear molecules!

9. Linear  Bent  3-Atom Molecules • Most likely possibilities are linear and bent

11. .. .. .. .. CO2 Lewis structure: O :: C :: O 4 bonding pairs But only2 regionsof electron density 180 apart so CO2 is linear

12. Lewis Structure of H2O = H:O:H .. Count up the electron pairs: H2O .. • 2 bonding pairs between O and the H’s • 2 nonbonding pairs on the O • 4 different regions of electron density

13. H2O • 4 electron pairs are 109 apart • nonbonding pairs spread out a bit more • squeeze the bonding pairs together • bent shape

15. 3-Atom Molecules • Triangular? Yes, ozone (O3) is triangular

16. .. .. F .. .. .. .. .. F .. .. BF3: Trigonal Planar • B has 3 valence electrons. It’s a very small atom. Each F has 7 valence electrons. Total = 3 X 7 + 3 = 24 : F : B These molecules don’t obey the octet rule - so aren’t likely to see them - 3 regions - FBF = 120 ..

17. 4-Atom Molecules • Two possibilities: • Trigonal Planar – in 1 plane • Trigonal Pyramidal

18. .. Lewis Structure: H : N : H .. NH3 H • 3 bonding pairs • 1 nonbonding pair • 4 regions • 109 apart

19. H Lewis structure = H : C : H .. H CH4 .. • 4 bonding pairs • 4 regions of electron density • Electron pairs are 109 apart

20. 4-Atom Molecules: Trigonal Planar Bond angles =120 All 4 atoms lie in the same plane You aren’t likely to see this because thesemolecules don’t obey the octet rule!

21. 4-atom molecule • Shape = trigonal pyramid • A bit less than 109

22. 4-Atom Molecules: AX3 The shape you are most likely to see

23. 5-Atom Molecules: AX4

24. 5 Atoms & Tetrahedrons Tetrahedral means 4 faces: - 1 atom in center - 4 atoms at the points

25. CH4 = a 5-atom molecule

26. CH4 = a 5-atom molecule

27. Summary of Molecular Shapes • Start with Lewis Structure! • Look at number of regions of electron density on central atom • Look at number of atoms bonded to central atom

28. Molecular Polarity • Look at the type of bonds in the molecule • Look at the shape of the molecule • A nonpolar molecular must: be symmetrical

29. Molecular Polarity • molecule is symmetric if: • electrical charge on 1 side = electrical charge at matching point on opposite side • the “pull” of one polar bond is offset by the “pull” of another polar bond • tug-of-war that can't be won!

30.  Nonpolar Symmetrical Molecules  Nonpolar

31. Asymmetric Molecules  Polar