1. 1 4.9 The Polarity of Covalent Molecules Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Chapter 4 Forces Between Particles

2. 2 Polar Molecules A polar molecule contains polar bonds. has a separation of positive and negative charge called a dipole indicated with  + and  -. has dipoles that do not cancel.  +  - H–Cl H — N — H dipole H dipoles do not cancel

3. 3 Nonpolar Molecules A nonpolar molecule contains nonpolar bonds. Cl–Cl H–H or has a symmetrical arrangement of polar bonds. O=C=O Cl Cl–C–Cl Cl dipoles cancel

4. 4 Determining Molecular Polarity STEP 1: Write the electron-dot formula. STEP 2: Determine the polarity of the bonds. STEP 3: Determine if dipoles cancel. Example: H 2 O.. H─O : H 2 O is polar │ H dipoles do not cancel

5. 5 Learning Check Identify each of the following molecules as 1) polar or 2) nonpolar. Explain. A. PBr 3 B. HBr C. Br 2 D. SiBr 4

6. 6 Solution Identify each of the following molecules as 1) polar or 2) nonpolar. Explain. A. PBr 3 1) pyramidal; dipoles don’t cancel; polar B. HBr1) linear; one polar bond (dipole); polar C. Br 2 2) linear; nonpolar bond; nonpolar D. SiBr 4 2) tetrahedral; dipoles cancel; nonpolar

7. 7 The electronegativity value indicates the attraction of an atom for shared electrons. increases from left to right going across a period on the periodic table. is high for the nonmetals with fluorine as the highest. is low for the metals. Electronegativity

8. 8 Some Electronegativity Values for Group A Elements Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Low values High values ` Electronegativity increases ` Electronegativity decreases

9. 9 A nonpolar covalent bond occurs between nonmetals. is an equal or almost equal sharing of electrons. has almost no electronegativity difference (0.0 to 0.4). Examples: Electronegativity Atoms Difference Type of Bond N-N 3.0 - 3.0 = 0.0 Nonpolar covalent Cl-Br 3.0 - 2.8 = 0.2 Nonpolar covalent H-Si2.1 - 1.8 = 0.3 Nonpolar covalent Nonpolar Covalent Bonds

10. 10 A polar covalent bond occurs between nonmetal atoms. is an unequal sharing of electrons. has a moderate electronegativity difference (0.5 to 1.7). Examples: Electronegativity Atoms DifferenceType of Bond O-Cl 3.5 - 3.0 = 0.5Polar covalent Cl-C 3.0 - 2.5 = 0.5Polar covalent O-S 3.5 - 2.5 = 1.0Polar covalent Polar Covalent Bonds

11. 11 Comparing Nonpolar and Polar Covalent Bonds Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

12. 12 Ionic Bonds An ionic bond occurs between metal and nonmetal ions. is a result of electron transfer. has a large electronegativity difference (1.8 or more). Examples: Electronegativity Atoms Difference Type of Bond Cl-K 3.0 – 0.8 = 2.2 Ionic N-Na 3.0 – 0.9 = 2.1 Ionic S-Cs2.5 – 0.7= 1.8 Ionic

13. 13 TABLE 4.14 Electronegativity and Bond Types

14. 14 Predicting Bond Types Table 4.15

15. 15 Use the electronegativity difference to identify the type of bond between the following: nonpolar covalent (NP), polar covalent (P), or ionic (I). A. K-N B. N-O C. Cl-Cl D. H-Cl Learning Check

16. 16 Use the electronegativity difference to identify the type of bond between the following: nonpolar covalent (NP), polar covalent (P), or ionic (I). A. K-N2.2ionic (I) B. N-O0.5 polar covalent (P) C. Cl-Cl0.0nonpolar covalent (NP) D. H-Cl0.9polar covalent (P) Solution

17. 17 Other Interparticle Forces Network Solid – A solid in which the lattice sites are occupied by atoms that covalently bonded to each other. Dipole-Dipole Interaction – The attractive force that exists between the positive end of one polar molecule and the negative end of another. Hydrogen Bonding – The result of attractive dipolar forces between molecules in which hydrogen atoms are covalently bonded to very electronegative elements (O, N, or F).

18. 18 Dipole alignment of polar covalent molecules

19. 19 Dipole-dipole interactions

20. 20 Snowflakes are a result of intermolecular forces Credit: Photo Researchers, Inc.

21. 21 Ice crystal lattice