1. Molecular Polarity

2. Depends on 2 factors. –Type of bonds in the molecule –Arrangement of bonds or shape of molecule

3. Diatomic Molecules Simplest case: Bond Polarity and Molecular Polarity are identical Larger Molecules – Have to look at how the bonds are arranged.

4. For larger molecules, look at the kind & arrangement of bonds to determine overall polarity of molecule. CH 4 has 4 bonds.

5. Polarity of Molecules A molecule may contain polar bonds, but not be polar! Depends on the geometry of the molecule. If molecule is symmetric, the “pull” of one polar bond is offset by the “pull” of another polar bond. –It’s a tug-of-war that no one can win!

6. Symmetric Molecules Contain at least one mirror plane.

7. Symmetric vs. Asymmetric CO 2 is nonpolar. Can’t tell the ends apart. H 2 O is bent. Electron cloud is lopsided. H 2 O is polar.

8. Polarity of CO 2 ? Draw an arrow along each bond pointing to the more electronegative atom. If the arrows cancel out, the molecule is NONPOLAR.

9. Polarity of H 2 O? The green arrows do not cancel out, so water is polar!

10. Symmetry of Larger Molecules CF 4 is fairly symmetric. Overall, it is nonpolar. The arrows cancel out.

11. Use the structural formula to predict Molecular Polarity! H H  C  H H  

12. Ethane = C 2 H 6 Ethene = C 2 H 4 Ethyne = C 2 H 2 These molecules are symmetric and the electron cloud is the same on both ends. Overall, they are nonpolar.

13. Molecular Polarity If you know the shape, you can use the arrow technique to determine the polarity. So how do you get the shape?

14. Polarity –Linear molecules Nonpolar if the ends are the same Polar if the ends are different –Tetrahedral Nonpolar if all 4 corners are the same Polar if even 1 corner is different –Trigonal Pyramids Always polar –Bent molecules Always polar