Attraction Between Molecules

1. Attraction Between Molecules Look at that electric personality! You sure are positive… do you want to hang out together? You are very attractive.

2. Intramolecular vs Intermolecular bonding . . . • Intramolecular forces are the forces of attraction which hold an individual molecule together • Non-polar covalent bonds • Polar covalent bonds • Intermolecular attractions are attractions between one moleculeand a neighboring molecule.

3. All molecules experience intermolecular attractions This is the force that describes the expected state of molecules: solids, liquids and gases • Solid: Intermolecular forces are strong • Liquid: Intermolecular forces are weak • Gas: Intermolecular forces are especially weak

4. Types of intermolecular forces: Collectively, these are called “van der Waals forces” after me, Johannes van der Waals • Dispersion forces • Dipole-dipole attraction • Hydrogen bonds

5. Dispersion forces are also called London Dispersion Forces, or just London Forces after me, Fritz London. Dispersion Forces • Dispersion Forces are weak forces caused by an instantaneous dipole, in which electron distribution becomes asymmetrical. • These are the ONLY forces of attraction that exist among noble gas atoms and nonpolar molecules.

6. Dispersion forces are temporary...

7. London Dispersion Forces

8. Dispersion forces increase in strength as the number of electrons increases & the molecular weight increases. • The more electrons you have and the bigger you are, the stronger the possible temporary dipoles. Would you expect neon or xenon to have a higher boiling point? Why?

9. Because xenon has stronger temporary dipoles, xenon atoms are "stickier“than neon atoms. Neon atoms willbreak away from each other at much lower temperatures than xenon atoms. • Compare the boiling points of the noble gases: helium -269°C neon -246°C argon -186°C krypton -152°C xenon -108°C radon -62°C

10. Dipole-Dipole Forces • Dipole-Dipole Attraction: occurs in molecules that have a permanent dipole.  i.e. polar molecules! • Molecules with dipoles orient themselves so that “+” and “-” ends of the dipoles are close to each other. • The strength of the attraction increases as the polarity increases. • These occur in addition to London Dispersion forces

11. Dipole Forces

12. A special type intermolecular bonding: Hydrogen Bonding • Hydrogen bond: A dipole-dipole attraction in which hydrogen is bound to a highly electronegative atom. (F, O, N) • Hydrogen bonding is the strongest of the intermolecular forces.

13. What types of molecules will hydrogen bond? • The molecule must have a hydrogen bonded to a very electronegative atom (F, O, N) • The hydrogen will form an intermolecular bond to the lone pairs of an electronegative atom (F, O, N) on a neighboring molecule. • For example, all of these molecules can hydrogen bond . . .NH3, H2O, HF. Why??

14. Hydrogen Bonding in Water . . . The perfect example

15. More Examples of Hydrogen Bonding. .