Basic Chemistry Copyright © 2011 Pearson Education, Inc.1

Chapter 10 Structures of Solids and Liquids

10.4 Attractive Forces between

Particles

Basic Chemistry Copyright © 2011 Pearson Education, Inc.2

Ionic Bonds

In ionic compounds, ionic bonds • are strong attractive forces• hold positive and negative ions together

Basic Chemistry Copyright © 2011 Pearson Education, Inc.3

Dipole-Dipole AttractionsIn covalent compounds, polar

molecules • exert attractive forces called

dipole-dipole attractions• form strong dipole attractions

called hydrogen bonds between hydrogen atoms bonded to F, O, or N, and other atoms that are strongly electronegative

Basic Chemistry Copyright © 2011 Pearson Education, Inc.4

Dipole-Dipole Attractions

Basic Chemistry Copyright © 2011 Pearson Education, Inc.5

Dispersion Forces

Dispersion forces are• weak attractions between nonpolar molecules• caused by temporary dipoles that develop

when electrons are not distributed equally

Basic Chemistry Copyright © 2011 Pearson Education, Inc.6

Comparison of Bonding and Attractive Forces

Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Mass and Boiling Points

Nonpolar covalent compounds such as alkanes have higher boiling points as

• the mass of the compounds increases• more dispersion forces form

7

Basic Chemistry Copyright © 2011 Pearson Education, Inc.8

Melting Points and Attractive Forces

• Ionic bonds require large amounts of energy to break apart. Ionic compounds have very high melting points.

• Hydrogen bonds are the strongest type of dipole-dipole attractions. They require more energy to break than other dipole attractions. Compounds with hydrogen bonds have moderate melting points.

Basic Chemistry Copyright © 2011 Pearson Education, Inc.9

Melting Points and Attractive Forces (continued)

• Dipole-dipole attractions are weaker than hydrogen bonds, but stronger than dispersion forces. They have low to moderate melting points.

• Dispersion forces are weak and little energy is needed to break them. Compounds with dispersion forces have the lowest melting points.

Basic Chemistry Copyright © 2011 Pearson Education, Inc.

Melting Points

10

Basic Chemistry Copyright © 2011 Pearson Education, Inc.11

Learning Check

Identify the major type of attractive force in each of the following substances:

1) ionic bonds 2) dipole-dipole attractions3) hydrogen bonds 4) dispersion forcesA. NCl3B. H2OC. Br2

D. KClE. NH3

Basic Chemistry Copyright © 2011 Pearson Education, Inc.12

Solution

Identify the major type of attractive force in each of the following substances:

1) ionic bonds 2) dipole-dipole attractions3) hydrogen bonds 4) dispersion forces 2 A. NCl3 3 B. H2O

4 C. Br2

1 D. KCl 3 E. NH3

Basic Chemistry Copyright © 2011 Pearson Education, Inc.13

Learning Check

Identify the compound in each pair that has the higher melting point. Explain.

A. NCl3 or NH3

B. HBr or Br2

C. KCl or HCl

Basic Chemistry Copyright © 2011 Pearson Education, Inc.14

Solution

A. NH3 would have a higher melting point than NCl3 because the hydrogen bonds in NH3 are stronger than the dipole-dipole attractions in NCl3.

B. HBr would have a higher melting point than Br2 because the dipole-dipole attractions in HBr are stronger than the dispersion forces in Br2

C. KCl would have a higher melting point than HCl because the ionic bonds in KCl are stronger than the dipole-dipole attractions in HCl.