Sample Study Sheet: Predicting Types of Attractions and Relative Strengths of Attractions Between Particles  

Tip-off You are asked to predict the relative strengths of attractions between particles of two substances, or you are asked a question that cannot be answered unless you know those relative strengths. (For example, you are asked to compare certain properties of substances, such as their relative boiling point temperatures.) 

General Steps -

Determine the type of attraction between the particles using the following steps (summarized in a Figure below).

Step #1: Classify each substance as either a metallic element, carbon in the diamond form, another nonmetallic element, an ionic compound, or a molecular compound.

• Metallic elements have metallic bonds. Go to Step #5.
• Carbon in the diamond formC(dia)has covalent bonds between its atoms. Go to Step #5.
• The atoms of the noble gases and the molecules of other nonmetallic elements are held together by London forces. Go to Step #5.
• The ions in ionic compounds are held together by ionic bonds. Go to Step #5.
• Continue to Step #2 for molecular compounds.

Step #2: For molecular compounds, draw the Lewis structure for the molecule.

• If the Lewis structure contains an O-H, N-H, or H-F bond, the attractions that are broken when the substance boils are hydrogen bonds enhanced by London forces. Go to Step #5.
• For other molecular compounds, go to Step #3.

Step #3: If there are no O-H, N-H, or H-F bonds, determine the polarity of the bonds.

• If there are no polar bonds, the molecules are nonpolar and London forces are broken when they boil. Go to Step #5.
• If there is at least one polar bond, go to Step #4(d).

Step #4: Predict whether the polar bonds are symmetrically or asymmetrically arranged.

• If the distribution of polar bonds is symmetrical and their dipoles equal, the molecules are nonpolar and London forces are broken when they boil.
• If the distribution of polar bonds is asymmetrical, or symmetrical with unequal dipoles, the molecules are polar and dipole-dipole attractions enhanced by London forces are broken when they boil.

Step #5: Although we cannot predict the relative strengths of attractions between all particles, we can apply one of the following guidelines to predict the relative strengths of attractions between some particles.

• For substances that contain particles of about the same size, the substances with chemical bonds (ionic, covalent, or metallic) have stronger attractions between particles than substances with intermolecular attractions (hydrogen bonds, dipole-dipole attractions, or London forces). Chemical bonds are generally stronger than intermolecular attractions.
• For molecular substances that contain molecules of about the same size, substances with hydrogen bonds have stronger attractions between the particles than substances with either dipole-dipole attractions or London forces, and substances with dipole-dipole attractions have stronger attractions between the particles than London forces. Hydrogen bonds are generally stronger than dipole-dipole attractions, which are generally stronger than London forces.
• For molecular substances that have the same type of intermolecular attraction, larger molecules form stronger mutual attractions. Larger molecules tend to have stronger attractions.

Click here to see an example of this task. 

Click here to see an exercise that will allow you to try this task yourself. 

Return to the Strengths of Attractions Page.