Polar Bonds and Molecules

1. Polar Bonds and Molecules 8.4 • Snow covers approximately 23 percent of Earth’s surface. Each individual snowflake is formed from as many as 100 snow crystals. The polar bonds in water molecules influence the distinctive geometry of snowflakes.

2. 8.4 Bond Polarity • Bond Polarity • How do electronegativity values determine the charge distribution in a polar bond?

3. 8.4 Bond Polarity • When the atoms in a bond pull equally (as occurs when identical atoms are bonded), the bonding electrons are shared equally, and the bond is a nonpolar covalent bond.

4. 8.4 Bond Polarity • The bonding pairs of electrons in covalent bonds are pulled by the nuclei.

5. 8.4 Bond Polarity • The chlorine atom attracts the electron cloud more than the hydrogen atom does.

6. 8.4 Bond Polarity • A polar covalent bond, known also as a polar bond, is a covalent bond between atoms in which the electrons are shared unequally. • The more electronegative atom attracts electrons more strongly and gains a slightly negative charge. The less electronegative atom has a slightly positive charge.

7. 8.4 Bond Polarity Notes from WB: as the difference in electronegativity between the atoms in the bond increases the ionic character of the bond increases and the covalent character decreases. Ionic bonds are generally formed between metals and nonmetals. WB gives the range for ionic bonds to be a difference of greater than or equal to 1.7 but you will need to consider the type of elements as well as the electronegativity values. HF, for example , has a polar covalent bond (not an ionic bond).

8. 8.4 Polar Molecules • Polar Molecules • What happens to polar molecules between a pair of oppositely charged metal plates?

9. 8.4 Polar Molecules • In a polar molecule, one end of the molecule is slightly negative and the other end is slightly positive. • A molecule that has two poles is called a dipolar molecule, or dipole.

10. 8.4 Polar Molecules • When polar molecules are placed between oppositely charged plates, they tend to become oriented with respect to the positive and negative plates.

11. 8.4 Polar Molecules • A hydrogen chloride molecule is a dipole.

12. Polar Molecules • Animation 10 Learn to distinguish between polar and nonpolar molecules.

13. 8.4 Attractions Between Molecules • Attractions Between Molecules • How do intermolecular attractions compare with ionic and covalent bonds?

14. 8.4 Attractions Between Molecules • Intermolecular attractions are weaker than either ionic or covalent bonds. • These attractions are responsible for determining whether a molecular compound is a gas, a liquid, or a solid at a given temperature.

15. 8.4 Attractions Between Molecules • Van der Waals Forces • The two weakest attractions between molecules are collectively called van der Waals forces, named after the Dutch chemist Johannes van der Waals (1837–1923).

16. 8.4 Attractions Between Molecules • Dipole interactions occur when polar molecules are attracted to one another.

17. 8.4 Attractions Between Molecules • Dispersion forces, the weakest of all molecular interactions, are caused by the motion of electrons. • The strength of dispersion forces generally increases as the number of electrons in a molecule increases.

18. What are van der waals forces? • http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtml • Hydrogen bonding video: • http://www.youtube.com/watch?v=LGwyBeuVjhU

19. 8.4 Attractions Between Molecules • Hydrogen Bonds • Hydrogen bonds are attractive forces in which a hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom.

20. 8.4 Attractions Between Molecules • Hydrogen Bonding in Water

21. 8.4 Attractions Between Molecules • The relatively strong attractive forces between water molecules cause the water to form small drops on a waxy surface.

22. 8.4 Intermolecular Attractions and Molecular Properties • Intermolecular Attractions and Molecular Properties • Why do network solids have high melting points?

23. 8.4 Intermolecular Attractions and Molecular Properties • Network solids (or network crystals) are solids in which all of the atoms are covalently bonded to each other. • Network solids consist of molecules that do not melt until the temperature reaches 1000°C or higher, or they decompose without melting at all.

24. 8.4 Intermolecular Properties and Molecular Properties • Melting a network solid would require breaking covalent bonds throughout the solid.

25. 8.4 Intermolecular Attractions and Molecular Properties • Diamond is an example of a network solid. Diamond does not melt. It vaporizes to a gas at 3500°C or above.

26. 8.4 Intermolecular Attractions and Molecular Properties • Silicon Carbide is a network solid. It has a melting point of about 2700°C.

27. Intermolecular Attractions and Molecular Properties • Simulation 8 Relate melting and boiling points to the strength of intermolecular forces.

28. 8.4 Intermolecular Attractions and Molecular Properties

29. 8.4 Section Quiz. • 8.4.

30. 8.4 Section Quiz. • 1. In a molecule, the atom with the largest electronegativity value • repels electrons more strongly and aquires a slightly negative charge. • repels electrons more strongly and aquires a slightly positive charge. • attracts electrons more strongly and aquires a slightly positive charge. • attracts electrons more strongly and aquires a slightly negative charge.

31. 8.4 Section Quiz. • 2. When polar molecules are placed between oppositely charged plates, the negative • molecules stick to the positive plates. • molecules stick to the negative plates. • ends of the molecules turn toward the positive plates. • ends of the molecules turn toward the negative plates.

32. 8.4 Section Quiz. • 3. Which of the following bond types is the weakest? • ionic bond • Van der Waals force • covalent bond • hydrogen bond