1. Properties of Covalent (Molecular) Substances

2. Poor conductors of heat & electricity in any phase. No charged particles! Low melting & boiling points: easy to pull molecules apart from each other. Majority of solids are soft Low H f and H v compared to ionic & metallic substances High vapor pressure compared to ionic & metallic substances

3. Properties Depend on strength of forces between “particles” or separate units. In covalent substances, the units are molecules.

4. Intermolecular Forces Forces between molecules = intermolecular forces 3 kinds: What are they?

5. Intermolecular Forces Dispersion forcesDispersion forces occur between nonpolar molecules. (Van der Waals) Dipole-dipole forcesDipole-dipole forces occur between polar molecules. Hydrogen bondingHydrogen bonding occurs between molecules with an H-F, H-O, or H-N bond. Intermolecular forces determine phase!

6. Weakest intermolecular force = dispersion force. Occurs between nonpolar molecules. 1) Monatomic molecules: He, Ne, Ar, Kr, etc. 2) Diatomics with 2 atoms of the same element. 3) Very symmetric molecules. Dispersion forces are stronger for larger molecules.

8. Dipole-dipole forces occur between molecules with a permanent charge separation, i.e., polar molecules. HCl and HBr are examples of polar diatomic molecules.

9. Hydrogen bonding influences properties!

10. Molecular Polarity For diatomics, the molecular polarity is the same as the bond polarity. and the shapeFor molecules with 3 or more atoms, the polarity depends on the types of bonds and the shape of the molecule!

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

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

13. Hydrogen bonding occurs between molecules containing an H-F, H-O, or H-N bond. It is the strongest intermolecular force.

14. Properties of Covalent (Molecular) Substances Poor conductors of heat & electricity in any phase. No charged particles! Low melting & boiling points: easy to pull molecules apart from each other. Majority of solids are soft Low H f and H v compared to ionic & metallic substances High vapor pressure compared to ionic & metallic substances

15. Melting pt, Boiling pt, H f and H v and vapor pressure depend on how hard it is to pull the particles apart. Weak intermolecular forces – it’s easy to pull them apart. Strong intermolecular forces – it’s hard.

16. Which substance has the strongest intermolecular forces? The weakest? Water Ether

17. 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!

18. Symmetric Molecules Contain at least one mirror plane.

19. 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.

20. 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.

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

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

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

24. 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.

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