SHAPES OF MOLECULES

1. SHAPES OF MOLECULES

2. REMINDER ABOUT ELECTRONS • Electrons have negative charges • Negative charges “repel” each other • In molecules, electrons want to get as far away from each other as possible • As a result, this repulsion of electrons leads to the shape of the molecule

3. SHAPE OF MOLECULES • There is a simple model used to determine the shape of molecules. • VSEPR (Valence Shell Electron Pair Repulsion) • A simple model that predicts the general shape of a molecule based on the repulsion between both the bonding and nonbonding electron clouds • Always based on the CENTRAL atom

4. DEFINITION • Electron cloud: Any type of bond (single, double or triple) or any set of unshared pairs of electrons. • Unshared pair of electron: any pair of electrons not involved in a covalent bond.

5. 2 ELECTRON CLOUDS • Example: CO2 • Count the number of electron clouds surrounding the central atom . . . . O=C=O · · ·· • There are 2 double bonds around the central Carbon (C) • Thus, there are 2 electron clouds

6. 2 ELECTRON CLOUDS • Electrons have a negative charge and repel each other • Thus, the electrons that maintain the bond will move as far away from each other as possible • If the central atom has only two electron clouds, it will have a linear shape

7. LINEAR • View the clip • NOTE: That the attached atoms are 180° apart from each other

8. 3 ELECTRON CLOUDS • Again count the number of electron clouds around the central atom (SO3) .. : O: .. | .. :0-S=O ·· ··

9. 3 ELECTRON CLOUDS • Again the electron clouds want to move as far from each other as possible • When the central atom has 3 electron clouds surrounding it, the molecule has a trigonal planar shape

10. TRIGONAL PLANAR • View the clip • NOTE: The attached atoms are 120° apart from each other

11. 3 ELECTRON CLOUDS • What happens if one of the electron clouds is an unshared pair of electrons • O3 • Do the Lewis dot structure for this in your notes

12. 3 ELECTRON CLOUDS • Notice the following: • You have one double bond • You have one single bond • You have one unshared pair of electrons • When you view a molecule, you can’t see the unshared pair of electrons • This creates a bent shape

13. BENT • View the clip • NOTE: That the attached atoms and unshared pair are 120° apart from each other • Since you can’t see the unshared pair, the molecule looks bent

14. 4 ELECTRON CLOUDS • Again count the number of electron clouds • CCl4 • Draw the Lewis dot structure in your notes

15. 4 ELECTRON CLOUDS • Same as before, the electron clouds want to get as far from each other as possible • When the central atom has 4 electron clouds surrounding it, you get a tetrahedral

16. TETRAHEDRAL • View the clip • NOTE: That the attached atoms are 109.5° apart from each other

17. 4 ELECTRON CLOUDS • What happens if you have 3 atoms bound to a central atom with one unshared pair • NH3 • Do the Lewis dot structure for this in your notes

18. 4 ELECTRON CLOUDS • Notice the following: • You have three single bonds • You have one unshared pair of electrons • When you view a molecule, you can’t see the unshared pair of electrons • This creates a pyramidal shape

19. PYRAMIDAL • View the clip • NOTE: That the attached atoms and unshared pair are 109.5° apart from each other • Since you can’t see the unshared pair, the molecule looks like a pyramid

20. 4 ELECTRON CLOUDS • What happens if you have 2 atoms bound to a central atom with two unshared pairs • H2O • Do the Lewis dot structure for this in your notes

21. 4 ELECTRON CLOUDS • Notice the following: • You have two single bonds • You have two unshared pairs of electrons • When you view a molecule, you can’t see the unshared pair of electrons • This creates a bent shape

22. BENT • View the clip • NOTE: That the attached atoms and unshared pair are 109.5° apart from each other • Since you can’t see the unshared pair, the molecule looks like a pyramid

23. TRY THESE • HANDOUT • For each of the following, write the Lewis structure and indicate the shape: • CBr4 • CS2

24. POLARITY OF MOLECULES • We’ve already discussed the difference between nonpolar, polar and ionic bonds (electronegativity difference) • Molecular shape is important for determining the polarity of a molecule. • Covalently bonded molecules can be polar or nonpolar based on the shape of the molecule

25. EXAMPLE • Let’s look at the shapes of • H2O • CF4 • First of all: • H-O bond of water has an electronegativity difference of 1.4 (polar covalent) • C-F bond of CF4 has an electronegativity difference of 1.5 (polar covalent)

26. EXAMPLE • Since both H2O and CF4 have polar covalent bonds, we would expect both molecules to be polar covalent • This is not the case • IN YOUR NOTES: Draw the molecular shape of both: • H2O • CF4

27. ANSWER – THINK OF TUG-O-WAR • Water is a bent molecule • As a result water is a polar molecule • You have a partial charge of σ- for O and σ+ for H • CF4 is a tetrahedral molecule • Because of the shape you have a nonpolar molecule • Even though you have partial charges, the charges cancel out because of the shape

28. WRITE WHICH SHAPES ARE POLAR/NONPOLAR? • Take a look at your handout that shows the shape of different molecules. • Which shapes do you think are polar? • Which shapes do you think are nonpolar?

29. ANSWER • Polar shapes (have lone pairs): • Bent • Pyramidal • Nonpolar shapes (do not have lone pairs): • Linear • Trigonal planar • Tetrahedral

30. TRY THE FOLLOWING • Determine the Lewis structure of the following. Are the molecules polar or nonpolar? • Cl2O • CO2 • NF3

31. ANSWER • Bent, polar molecule • Linear, nonpolar molecule • Pyramidal, polar molecule