1. Chapter 10 Properties of Solids and Liquids
10.4 Attractive Forces in Compounds
Learning Goal Describe the attractive forces between ions, polar covalent molecules, and nonpolar covalent molecules.

2. Attractive Forces in Compounds
Attractive forces between molecules and ions hold them close together in liquids and solids.
Solids melt, and liquids boil when the attractive forces between molecules are broken.
Attractive forces between molecules can be
dipole−dipole attractions
hydrogen bonding
dispersion forces

3. Dipole-Dipole Attractions
Polar molecules are attracted to each other by dipole-dipole attractions
when the positive end of one dipole is attracted to the negative end of a second dipole
such as the attractive forces between two molecules of H—Cl

4. Hydrogen Bonds
Hydrogen bonds, especially strong dipole-dipole attractions, occur between
polar molecules containing hydrogen atoms bonded to very electronegative atoms such as fluorine (F), nitrogen (N), and oxygen (O)
a hydrogen atom with a partial positive charge attached to N, O, or F and a partial negative charge on N, O, or F

5. Dispersion Forces
Dispersion forces, very weak attractive forces that occur between nonpolar molecules,
occur when movement induces a temporary distortion of the electrons in a molecule, creating a temporary dipole
make it possible for nonpolar molecules to exist as liquids and solids

6. Bonding and Attractive Forces

7. Learning Check
Indicate the major type of molecular interaction such as dipole-dipole attractions, hydrogen bonds, or dispersion forces expected between each of the following: A. NF3 B. Cl2 C. HF

8. Solution
Indicate the major type of molecular interaction such as dipole-dipole attractions, hydrogen bonds, or dispersion forces expected between molecules of:
A. NF3 dipole-dipole attractions
B. Cl2 dispersion forces
HF hydrogen bonds

9. Size, Mass, and Boiling Points
As the size and mass of similar types of molecules increase,
the attractive forces between the molecules also increase
there are more electrons available to produce stronger temporary dipoles
These increased attractive forces increase the boiling points of these molecules.

10. Molar Mass and Boiling Points, Alkanes

11. Attractive Forces and Melting Points
The melting point of a substance is related to the strength of the attractive forces between its particles. Molecules with
weaker dispersion forces have lower melting points; it takes less energy to break the dispersion forces
stronger dipole-dipole forces, or hydrogen bonds, require more energy to break the forces between them

12. Highest Melting Points
The highest melting points occur in ionic compounds that have very strong attractive forces between positive and negative ions. Ionic compounds require large amounts of energy to break these forces and melt the substance.

13. Melting Points of Selected Substances

14. Chemistry Link to Health
Biological molecules such as proteins have many different functions. They are needed for
structural components such as cartilage, muscles, hair, and nails
the formation of enzymes to regulate biological reactions
transport of oxygen in blood and muscles

15. Chemistry Link to Health
Proteins are composed of building blocks called amino acids. Amino acids have a central carbon atom bonded to
an –NH3+ from an amine and a –COO− from a carboxylic acid
an H atom and a side chain called an R group

16. Chemistry Link to Health
Different amino acids have different R groups. The R group on alanine is a –CH3 group.
(a) The ionized form of alanine contains —NH3+, —COO–, H, and a —CH3 group. (b) The structure of alanine represented as a ball-and-stick model.

17. Chemistry Link to Health
Examples of other amino acids with their side chains include:

18. Chemistry Link to Health
In the primary structure of proteins, amino acids are linked by peptide bonds between the amine and the carboxylic acid groups.

19. Chemistry Link to Health
In a higher level structure of proteins, hydrogen bonds are formed to fold the proteins into different shapes, such as an alpha helix.

20. Chemistry Link to Health
Hydrogen bonding in proteins also occurs between the polar side chains on amino acids on the outer surface of the protein, and the –OH and –H of polar water molecules.

21. 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

22. Solution
Identify the compound in each pair that has the higher melting point. Explain. A. NCl3 or NH3 NH3 has a higher melting point than NCl3 because the hydrogen bonds in NH3 are stronger than the dipole-dipole attractions in NCl3.

23. Solution
Identify the compound in each pair that has the higher melting point. Explain. B. HBr or Br2 HBr would have a higher melting point than Br2 because the dipole-dipole attractions in HBr are stronger than the dispersion forces in Br2.

24. Solution
Identify the compound in each pair that has the higher melting point. Explain. C. KCl or HCl KCl would have a higher melting point than HCl because the ionic bonds in KCl are stronger than the dipole-dipole attractions in HCl.