Study Sheet: Predicting Types of Attractions and Relative Strengths of Attractions
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
Step #1: Classify each substance as either a metallic element,
carbon in the diamond form, another nonmetallic element, an ionic compound, or a
- 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
- 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
- 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.
here to see an example of this task.
here to see an exercise that will allow you to try this task yourself.
the Strengths of Attractions Page.