1. 8.4 Polar Bonds and Molecules > 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 8 Covalent Bonding 8.1 Molecular Compounds 8.2 The Nature of Covalent Bonding 8.3 Bonding Theories 8.4 Polar Bonds and Molecules

2. 8.4 Polar Bonds and Molecules > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The character of the molecule depends on the kind and number of atoms joined together. These features, in turn, determine the molecular properties. Bond Polarity Covalent bonds differ in terms of how the bonded atoms share the electrons.

3. 8.4 Polar Bonds and Molecules > 3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Bond Polarity The bonding pairs of electrons in covalent bonds are pulled between the nuclei of the atoms sharing the electrons. The nuclei of atoms pull on the shared electrons, much as the knot in the rope is pulled toward opposing sides in a tug-of-war.

4. 8.4 Polar Bonds and Molecules > 4 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. When the atoms in the bond pull equally (as occurs when identical atoms are bonded), the bonding electrons are shared equally, and each bond formed is a nonpolar covalent bond. Bond Polarity The bonding pairs of electrons in covalent bonds are pulled between the nuclei of the atoms sharing the electrons.

5. 8.4 Polar Bonds and Molecules > 5 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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.

6. 8.4 Polar Bonds and Molecules > 6 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The higher the electronegativity value, the greater the ability of an atom to attract electrons to itself. Bond Polarity

7. 8.4 Polar Bonds and Molecules > 7 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Bond Polarity Electronegativity Differences and Bond Types Electronegativity difference range Most probable type of bondExample 0.0–0.4Nonpolar covalentH—H (0.0) 0.4–1.0Moderately polar covalent δ+ δ– H—Cl (0.9) 1.0–2.0Very polar covalent δ+ δ– H—F (1.9) >2.0IonicNa + Cl – (2.1) Describing Polar Covalent Bonds The electronegativity difference between two atoms tells you what kind of bond is likely to form.

8. 8.4 Polar Bonds and Molecules > 8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

9. 8.4 Polar Bonds and Molecules > 9 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 8.3 Identifying Bond Type Which type of bond (nonpolar covalent, moderately polar covalent, very polar covalent, or ionic) will form between each of the following pairs of atoms? a. N and H b. F and F c. Ca and Cl d. Al and Cl

10. 8.4 Polar Bonds and Molecules > 10 Complete the table Calculated polarityBond type Si and O Mg and N Fe and S O and O H and H N and O S and F K and I As and Cl Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

11. 8.4 Polar Bonds and Molecules > 11 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. In a polar molecule, one end of the molecule is slightly negative, and the other end is slightly positive. Bond Polarity Describing Polar Covalent Molecules The presence of a polar bond in a molecule often makes the entire molecule polar.

12. 8.4 Polar Bonds and Molecules > 12 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A molecule that has two poles is called a dipolar molecule, or dipole Bond Polarity Describing Polar Covalent Molecules The water molecule has two polar bonds.

13. 8.4 Polar Bonds and Molecules > 13 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. When polar molecules are placed between oppositely charged plates, they tend to become oriented with respect to the positive and negative plates. Bond Polarity Describing Polar Covalent Molecules

14. 8.4 Polar Bonds and Molecules > 14 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The water molecule also has two polar bonds. Bond Polarity Describing Polar Covalent Molecules However, the water molecule is bent rather than linear. Therefore, the bond polarities do not cancel and a water molecule is polar.

15. 8.4 Polar Bonds and Molecules > 15 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Attractions Between Molecules How do the strengths of intermolecular attractions compare with ionic and covalent bonds? Attractions Between Molecules

16. 8.4 Polar Bonds and Molecules > 16 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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. Van der Waals forces consist of dipole interactions and dispersion forces. Attractions Between Molecules

17. 8.4 Polar Bonds and Molecules > 17 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Van der Waals Forces Dipole interactions occur when polar molecules are attracted to one another. Attractions Between Molecules The electrical attraction occurs between the oppositely charged regions of polar molecules. Dipole interactions are similar to, but much weaker than, ionic bonds.

18. 8.4 Polar Bonds and Molecules > 18 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The slightly negative region of a polar molecule is weakly attracted to the slightly positive region of another polar molecule. Attractions Between Molecules Van der Waals Forces Dipole interactions are similar to, but much weaker than, ionic bonds.

19. 8.4 Polar Bonds and Molecules > 19 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Van der Waals Forces Dispersion forces, the weakest of all molecular interactions, are caused by the motion of electrons. Attractions Between Molecules They occur even between nonpolar molecules.

20. 8.4 Polar Bonds and Molecules > 20 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Van der Waals Forces Dispersion forces, the weakest of all molecular interactions, are caused by the motion of electrons. Attractions Between Molecules When the moving electrons happen to be momentarily more on the side of a molecule closest to a neighboring molecule, their electric force influences the neighboring molecule’s electrons to be momentarily more on the opposite side.

21. 8.4 Polar Bonds and Molecules > 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Van der Waals Forces Dispersion forces, the weakest of all molecular interactions, are caused by the motion of electrons. Attractions Between Molecules The strength of dispersion forces generally increases as the number of electrons in a molecule increases.

22. 8.4 Polar Bonds and Molecules > 22 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Van der Waals Forces Attractions Between Molecules Fluorine and chlorine have relatively few electrons and are gases at ordinary room temperature and pressure because of their especially weak dispersion forces. Bromine molecules therefore attract each other sufficiently to make bromine a liquid under ordinary room temperature and pressure. Iodine, with a still larger number of electrons, is a solid at ordinary room temperature and pressure.

23. 8.4 Polar Bonds and Molecules > 23 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Hydrogen Bonds Attractions Between Molecules The positive region of one water molecule attracts the negative region of another water molecule.

24. 8.4 Polar Bonds and Molecules > 24 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Hydrogen Bonds Attractions Between Molecules This relatively strong attraction, which is also found in hydrogen-containing molecules other than water, is called a hydrogen bond. Hydrogen bond

25. 8.4 Polar Bonds and Molecules > 25 Hydrogen bonds http://www.youtube.com/watch?v=lkl5cbfqFR Mhttp://www.youtube.com/watch?v=lkl5cbfqFR M Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

26. 8.4 Polar Bonds and Molecules > 26 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Hydrogen Bonds Attractions Between Molecules A hydrogen bond has about 5 percent of the strength of the average covalent bond. Hydrogen bonds are the strongest of the intermolecular forces. They are extremely important in determining the properties of water and biological molecules.

27. 8.4 Polar Bonds and Molecules > 27 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The diversity of physical properties among covalent compounds is mainly because of widely varying intermolecular attractions. Intermolecular Attractions and Molecular Properties

28. 8.4 Polar Bonds and Molecules > 28 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. In most solids formed by molecules, only the weak attractions between molecules need to be broken. Intermolecular Attractions and Molecular Properties The melting and boiling points of most compounds composed of molecules are low compared with those of ionic compounds.

29. 8.4 Polar Bonds and Molecules > 29 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Most of these very stable substances are network solids (or network crystals), solids in which all of the atoms are covalently bonded to each other. –Melting a network solid would require breaking covalent bonds throughout the solid. Intermolecular Attractions and Molecular Properties A few solids that consist of molecules do not melt until the temperature reaches 1000°C or higher.

30. 8.4 Polar Bonds and Molecules > 30 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Intermolecular Attractions and Molecular Properties Diamond is an example of a network solid. Each carbon atom in a diamond is covalently bonded to four other carbons, interconnecting carbon atoms throughout the diamond. Diamond does not melt; rather, it vaporizes to a gas at 3500°C and above.

31. 8.4 Polar Bonds and Molecules > 31 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. This table summarizes some of the characteristic differences between ionic and covalent (molecular) substances. Characteristics of Ionic and Molecular Compounds CharacteristicIonic CompoundMolecular Compound Representative unitFormula unitMolecule Bond formation Transfer of one or more electrons between atoms Sharing of electron parts between atoms Type of elementsMetallic and nonmetallicNonmetallic Physical stateSolidSolid, liquid, or gas Melting pointHigh (usually above 300°C)High (usually below 300°C) Solubility in waterUsually highHigh to low Electrical conductivity of aqueous solution Good conductorPoor to nonconducting Intermolecular Attractions and Molecular Properties

32. 8.4 Polar Bonds and Molecules > 32 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. When different atoms bond, the more electronegative atom attracts electrons more strongly and acquires a slightly negative charge. Intermolecular attractions are weaker than either an ionic or a covalent bond. The diversity of physical properties among covalent compounds is mainly because of widely varying intermolecular attractions. Key Concepts

33. 8.4 Polar Bonds and Molecules > 33 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. nonpolar covalent bond: a covalent bond in which the electrons are shared equally by the two atoms polar covalent bond (polar bond): a covalent bond between atoms in which the electrons are shared unequally polar molecule: a molecule in which one side of the molecule is slightly negative and the opposite side is slightly positive Glossary Terms

34. 8.4 Polar Bonds and Molecules > 34 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. dipole: a molecule that has two poles, or regions with opposite charges van der Waals forces: the two weakest intermolecular attractions—dispersion interactions and dipole forces dipole interactions: intermolecular forces resulting from the attraction of oppositely charged regions of polar molecules Glossary Terms

35. 8.4 Polar Bonds and Molecules > 35 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. dispersion forces: attractions between molecules caused by the electron motion on one molecule affecting the electron motion on the other through electrical forces; these are the weakest interactions between molecules hydrogen bonds: 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 network solid: a solid in which all of the atoms are covalently bonded to each other Glossary Terms