Teaching Intermolecular Attractive Forces

by

John Gelder
Professor of Chemistry
Oklahoma State University

To thoroughly cover intermolecular attractive forces requires knowledge in a wide range of topics. Including;

Ionic and covalent compounds
Coulomb's law
Lewis structures
Molecular geometry
Polarity
Atomic structure


I. Ionic Compounds

The discussion could begin with the consideration of ionic compounds. Write the formula of an ionic compound and ask students what anyone remembers about ionic compounds.

One could ask leading questions like;

Question

Response

Looking at a chemical formula, how do you know a compound is ionic?

Formula contains a metal and a nonmetal, and watch out for ionic compounds with the ammonium ion.

What do ionic compounds contain?

Cations/positively charged ions and anion/negatively charged ions.

Can someone define an ionic bond?

Electrostatic attraction between oppositely charged ions.

Here is a diagram of an ionic compound represented by different sized spheres.

Question

Response

Ask students what they see in the diagram above?

Students should respond that they see two different colored spheres; small green spheres and larger bluish colored spheres. Note it is important to begin with the simplest, obvious observations.

Looking at the diagram of an ionic compound, who is the cation and who is the anion?

The green sphere is the cation (loss of electrons should produce a smaller sphere for a cation.) The bluish sphere is the anion (gain of electrons should produce a larger sphere for the anion.) NOTE: my students usually claim the smaller sphere is the anion and the larger sphere the cation.

Ask the phase of the ionic compound you are discussing/every ionic compound?

Solid, due to the strong electrostatic attractive forces. Cations are surrounded by six anions and anions are surrounded by six cations.

What are some melting points for an ionic compound? Very high? Very low?

Students will not know any melting points, but they should know the melting point for an ionic compound is high. NaCl melts at 801 degrees Celsius; KCl melts at 770 degrees Celsius

Looking at this model of an ionic compound will establish an image in their minds that ionic compounds consist of alternating cations and anions that are strongly attracted to each other due to the full positive and negative charges (coulombic attractions).

II. Covalent Compounds

Now try a parallel discussion of covalent compounds, asking about the formula of a covalent compound.

Question

Response

How do you recognize covalent compounds from their formulas?

In general, the elements in the formula for a covalent compound are nonmetals.

What kind of bonds are in covalent compounds?

Covalent bonds.

What is a covalent bond?

Atoms sharing electrons as a result of the overlap of atomic or hybrid orbitals.

Ask the phase of the covalent compounds?

Solid, liquid and gas.

What are some melting points and boiling points for covalent compounds? Very high? Very low?

Students will not know many melting points or boiling points, except for water, H2O.

III. Polarity

Below is a figure depicting the distribution of electron density in the F2 molecule (on the left) and the HF molecule (on the right).

 

Question

Response

Which image, F2 or HF, do you believe represents a symmetric distribution of electron density?

The image of F2 shows a symmetric distribution of electrons.

Which image, F2 or HF, do you believe represents a asymmetric distribution of electron density?

The image of HF shows a symmetric distribution of electrons.

In the image of HF what do you think the color blue and color red represent?

H-F is a polar covalent molecule due to the dfference in electronegativity between hydrogen and fluorine. This diference is what gives rise to the polar covalent bond. So the blue color is due to the partial positive charge on hydrogen and the red color is due to the partial negative charge on fluorine.

In the figure of HF label each atom with a symbol (δ+ or δ-) to identify the atom with partial positive charge (δ+) and the atom with partial negative charge (δ-).

 

In the diagram of HF above an arrow (vector) has been drawn below the molecular structure. What do you think this arrow represents? Write a short explanation of what the arrow symbolizes. In your explanation use some or all of the following terms: covalent bond, nonpolar covalent bond, polar covalent bond, equal sharing of electrons, unequal sharing of electrons, electronegativity, difference in electronegativity, charge distribution, partial positive charge, partial negative charge, dipole moment.

 

IV. Boiling Points for the Noble Gases (Group VIII)

 

In the next section boiling point data will be displayed graphically with the goal of trying to get students to invent some different intermolecular attractive forces between atoms and/or molecules in liquids and solids.

Now look at some boiling points of some different substances. In Figure I are the boiling points of some noble gases (Group VIII or 18).

Figure I. Boiling points for the Noble Gases

Question

Response

Have the students describe what they see.

They should see the y-axis is boiling point, but also that the scale is in degrees Celsius and the x-axis is number of electrons/electron levels. So students should look at the x-axis as either the total number of electrons or the level of the valence electrons. So for He there are 2 electrons and valence electrons in the n=1 level; for Ne there are 10 electrons and valence electrons in the n=2 level; etc.

Now ask if they see any pattern in the boiling points of the elements?

Looking for the boiling point increases as you go down the group. Once they see the trend is for the bp to increase going down the group. Ask about the number of electrons in each of the elements. Someone is likely to say the boiling point increases as the molar mass increases, and if that is stated you need to indicate that that relationship has nothing to do with the trend in boiling points. Gravity has no connection with boiling point of substances.

What trend do they see with electrons?

Students should recognize that moving from helium to neon, etc the number of electrons increases. There is no explanation for that ..yet, but we will discuss it below in London Dispersion forces. Also mention that the electrons in krypton are in the n=4 level so they are very far from the nucleus compared to the electrons in helium (n=2 level), so the krypton atom has electrons that take up a large volume of space.

One could also ask about the polarity of the elements in this group?

All of the elements in Group VIII are nonpolar. At this point this is a factoid that we want students to have in the back of their minds.

So now students have a connection between the importance of the number of electrons and the boiling point. There is also the connection that all of the members of this group are nonpolar substances.

V. Boiling Points of the Group IV Hydrides

Now let’s look at another set of substances. In Figure II are the boiling points of some Group IV hydrides along with the boiling points of the members of Group VIII.

Figure II: Adding the boiling point for the Group IV hydrides

Question

Response

Ask students what trend do they see?

Here students should indicate that the boiling point of the compounds increases with increasing number of electrons. They should also see that the compounds contain elements in Group IV as the central atom and the trend is for increasing boiling point with increasing electrons. Also they should note that the pattern of increasing boiling point is similar to the pattern seen in the noble gases.

Are the members of the Group IV hydrides polar or nonpolar?

Nonpolar, however you may need to remind them what polarity is and how important molecular geometry is for predicting polarity. Then remind them about the noble gases, were they polar or nonpolar? So polarity may help explain some of the similarity in the boiling point behavior between noble gases and Group IV hydrides.

 

One could also note that all of the boiling points for the Group IV hydrides are displaced to higher temperature, but at this point, no explanation for that observation should be provided.

 

That substances exist in liquids and solids can only be explained in terms of some type of interaction between the particles in those phases. Since the trend for the nonpolar particles appears to depend on the number of electrons/the levels the valence electrons occupy we believe it is just these electrons and their behavior that can be used to understand the interaction that allows liquids and solids to exist.

What type of intermolecular attractive force explains the other trend that is observed in the boiling point diagram in Figure II? We discussed the boiling point trend in terms of the number of electrons in the molecule. How do electrons in a nonpolar molecule contribute to an intermolecular attractive force?

When looking at the movie ask the students what do they see? (See yellow spheres that could represent a noble gas atom or a nonpolar molecule. The important point is the spherical shape represents the volume occupied by the electrons in the noble gas atom or molecule. That the shape is spherical, symmetrical, suggests that the electrons are symmetrically distributed in the atom or molecule.)

So the yellow shape is representative of the electron distribution in the atom. NOTE: there is a moment in the movie where one of the yellow spheres turns many different colors, and then rotates around to reveal a look on the inside of the atom that has been sliced in half. This particular section of the movie was to reveal the symmetric distribution of electrons inside the shape. Unfortunately this brief section does not show up very well.

The next change in the movie shows one of the yellow spheres in a different shape. Ask students what might have happened to cause the shape of the electron density around the atom to change? (Since the shape is representative of the electron distribution, a different shape means that the electrons for that atom are no longer symmetrically distributed. Notice how the shape rotates to reveal the electron distribution. Also to help clarify, see the δ+ (partial positive charge) and δ- (partial negative charge) that is super-imposed over the atom with the asymmetric distribution of electrons.) Now watch how the single atom with the asymmetric distribution of electrons affects the atoms near it. Notice how the side of the atom with the partial positive charge induces a partial negative charge on the side of the atom adjacent to the original atom. This happens to many of the atoms near the first atom. Then in the next instant the asymmetric distribution of electrons changes. What this model has just demonstrated is what is referred to as an instantaneous dipole (a dipole that does not last very long). A nonpolar substance has no permanent dipole, however, a nonpolar atom or molecule can have an instantaneous dipole. Instantaneous dipoles arise when the normal symmetric distribution of electrons is distorted for an instant, resulting in an instantaneous dipole. For nonpolar molecules containing atoms from the first and second period the only intermolecular attractive force that can occur is London dispersion forces, and the London dispersion force is the weakest IMAF when compared to dipole dipole forces or hydrogen bond.

 

Now let’s look at another set of substances. In Figure III are the boiling points of some Group V hydrides (PH3, AsH3, SbH3).

Figure III : Adding the boiling point for the Group V hydrides

Question

Response

What is similar and what is different about the trend in boiling points for the Group V hydrides, compared to Group IV hydrides and the noble gases?

Students should indicate that the boiling point of the compounds increases with increasing number of electrons just like Group IV and the noble gases.

Ask if anyone remembers the polarity of the members of the Group V hydrides ?

They are all polar.

 

Here is a ball-and-stick model of PH3 showing where the partial positive charge and partial negative charge reside in the molecule. Since PH3 is polar and it has a permanent dipole. Molecules with permanent dipoles exhibit an intermolecular attractive force as shown below.

 

The dipole dipole intermolecular attractive force is found in all polar molecules.

 

It appears that the trend in boiling points for this set of Group V hydrides, even though each member is polar, follows the same trend as the nonpolar atoms/molecules discussed earlier. One can reasonably conclude that not only are London Dispersion forces present, but they are the most important intermolecular attractive force for this set of molecules.

Now let’s look at another set of substances. In Figure IV are the boiling points of some Group VII hydrides. Ask the students for the formulas of the Group VII hydrides? (They are HCl, HBr, and HI.)

Figure IV: Adding the boiling point for the Group VII hydrides

Question

Response

What is similar and what is different about the trend in boiling points for the Group VII hydrides, compared to Group IV and V and the noble gases?

Students should indicate that the boiling point of the compounds increases with increasing number of electrons just like Group IV and Group V and the noble gases. However, the hydride with the fewest number of electrons, HF is similar to NH3, as it has the highest boiling point in the group.

Ask if anyone remembers the polarity of this compounds?

They are all polar.

Below is a ball-and-stick model of HCl showing where the partial positive charge and partial negative charge reside in the molecule. Since HCl is polar and it has a permanent dipole. Molecules with permanent dipoles exhibit an intermolecular attractive force as shown below.

The dipole dipole intermolecular attractive force is found in all polar molecules.

It appears that the trend in boiling points for this set of Group VII hydrides, even though each member is polar, follows the same trend as the nonpolar atoms/molecules discussed earlier. One can reasonably conclude that not only are London Dispersion forces present, but they are the most important intermolecular attractive force for this set of molecules.

Can a student summarize the observations on the trend in boiling points for the four different groups (Group VIII, IV, V and VII) of substances and describe the two types of intermolecular attractive forces?

The trend in boiling points for the substances with the higher number of electrons is the same in all four groups, independent of whether the substance is polar or nonpolar. There are two difference types of intermolecular attractive forces occuring between molecules. London dispersion forces and dipole dipole forces. LDF occur in all substances and are due to the instantaneous dipoles that result from asymmetric distributions of electrons. Dipole dipole forces occur between polar molecules and are due to permanent dipoles in the molecule.

 

Again one could use Figure IVA to have students predict the boiling point for HF, rather than just showing the boiling point as provided in Figure IV.

 

Figure IVA: Adding the boling point for the Group VII hydrides (without the boiling point of HF)

Now before asking the questions associated with Figure IV, begin with the question asking students to predict the boiling point of HF.

Students should predict -100 C or something close.

Oops, the actual boiling point for HF is .

Clearly, since the boiling point for HF is considerably higher compared to the other members there must be something going on with molecules of HF that is different compared to molecules of the other Group V compounds.

Finally let’s consider another set of substances. The Group VI hydrides. But before displaying Figure V look at the questions below. The goal should be to get students to predict the trend in boling points of the Group VI hydrides. It would be OK to provide Figure IV to help guide the students to the prediction.

Question

Response

What is the formula for the members of the Group VI hydrides?

H2O, H2S, H2Te, H2Se

Ask if anyone can predict the trend in boiling points for each member of the group?

Everyone should recall the boiling point for H2O, then they should predict that for H2S the boiling point drops substantuell and the then begins to increase gradually. It is not important that they know the exact boiling points for H2S, H2Te, H2Se, just that they know the pattern.

After allowing time to predict show Figure V.

Figure V: Adding the boling point for the Group VI hydrides

Based on the boiling point data we can make the claim that there are some trends in the boiling points of the Nobel gases and the Group IV, V, VI, VII hydrides. One of those trends seems to be the connection between the number of electrons and the boiling point. This trend is observed in ALL of the higher members of the groups. The other trend is observed in the member with the fewest number of electrons in Groups V, VI and VII. The boiling points of these members are usually high, compared to the members with the fewest number of electrons in Groups VIII and IV.

At this point there might be some different paths that teachers could take in their discussion. One such path will be to discuss the hydrogen bond attractive force between water molecules, then dispersion forces, and finally dipole dipole forces. This order is suggested since it parallels the boiling point data discussed above.


Show a model of the water molecule using a surface that shows the asymmetric distribution of electrons. This is shown in Models 360.

 

 

This image of a water molecule was generated by entering 'water' in the Search field of Models 360. Then selecting, under the Display section, Molecular Electrostatic Potential, and then clicking on the radio button for MEP on isopotential surface. What we are seeing are colors that represent the electrostatic potential in a water molecules. The red color denotes a partial negative charge and the blue denotes a partial positive charge. This electrostatic potential is due to the polar bonds in H2O. The O-H bond is polar due to the difference in electronegativity between the oxygen atom and the hydrogen atom.

Here are two water molecules using a ball and stick model. Since water is polar I've added the partial positive and partial negative charges as indicated in the electrostatic potential surface as depicted using colors in the image above. The dashed line shows the electrostatic attraction (opposite charges) that could result from adjacent water molecules. Attractions between partial charges are not as strong as between full charges as experienced between cations and anions.

See the simulation below for the dynamic interaction of the water molecules. Be sure at some point to select the slow motion view.

Show a picture of water in Models360 and describe the electrostatic potential image. Discuss electronegativity and how we understand that the electrons in the O-H bonds are not shared equally, and the molecular geometry has the O-H bonds at an angle resulting in the polar nature of water molecule. Use partial charges on the H atoms and the O atoms. Explain what the lower case delta represents, that is that the charge on the H atoms and the O atoms are not the same magnitude as the charge on an ion in an ionic compound. Yet if two water molecules are next to each other they could be attracted to each other. The attraction, is electrostatic in nature, and, (this is very important) the electrostatic attraction is between water molecules. This type of attraction, between molecules, is called an intermolecular attraction. In this case that intermolecular attraction is given the name, hydrogen bond.


The simulation above was developed by the Concord.org. They have a collection of fantastic simulations, some of which are very useful when discussing intermolecular attractive forces. As the simulation plays try showing the partial charges, and clicking on the Slow Motion checkbox. Discuss what is happening with your students and be sure emphasize the intermolecular attractive force called hydrogen bond is an electrostatic attraction between molecules of water, as depicted inthe static representation above the simulation.

Looking back at Figure V, H2O, NH3 and HF all had boiling points that were unusually high. Also when we look at all three molecules they are very polar. Looking at the electrostatic potential surfaces for NH3 and HF we can see that.

Here are the electrostatic potential surfces for, NH3 and HF. Remember red denotes partial negative charge and blue, partial positive charge.

NH3

and

NH3

and

HF

Fluorine, oxygen and nitrogen have the largest electronegativities of the elements in the periodic table making the O-H, N-H and F-H bonds very polar. Hydrogen bonds are the most important intermolecular attractive force for all three compounds.

Hydrogen bonds are the most important intermolecular attractive force occurring in liquid H2O, NH3 and HF. Using at least two molecules, draw the intermolecular attractive hydrogen bond that occurs in NH3 and HF.

Students should show:

and

Here is a simulation that allows the selection of either polar or nonpolar molecules and their interaction.

Here is another simulation that allows a different perspective towards the intermolecular attraction between polar and nonpolar molecules.

As we see in the last two simulations there is a difference between the intermolecular attractive forces between polar and nonpolar molecules. At this point the only polar molecules we have discussed are those polar molecules that exhibit hydrogen bond intermolecular attractive forces. The hydrogen bond intermolecular attractive force only occurs between molecules that contain an O-H, N-H or F-H group. However, there is one more intermolecular attractive force that can occur in polar molecules, it is called dipole dipole intermolecular attractive force. Dipole dipole intermolecular attractive forces occur in all polar molecules. Let's consider a few polar molecules; CH2F2 and HCl.

Here are the electrostatic potential surfaces for, CH2F2 and HCl. Remember red denotes partial negative charge and blue, partial positive charge.

and

 

Here is the model of the dipole dipole interactions between CH2F2 molecules and between HCl molecules.

Here the dipole dipole interaction between molecules of CH2F2 and molecules of HCl are shown.

and

As one can see the electrostatic attraction between polar molecules and the hydrogen bonding electrostatic attraction between molecules look very similar. However, the hydrogen bonding intermolecular attractive forces between molecules is stronger compared to the dipole dipole intermolecular attractive forces between molecules. Also notice that in the ball and stick models a dashed line is used to identify the intermolecular attractive force between molecules. This is different compared to the solid line we use in Lewis structures to represent a covalent bond. The dashed line helps communicate that the intermoelcular attractive forces are weaker forces between molecules, compared to the covalent bonds between atoms in a covalent compound.

________________________________________________________

Link for boiling point data : http://intro.chem.okstate.edu/1515SP02/Lecture/Chapter12/BPTrends.html
The above file on boiling point trend is a macromedia file: best chance of viewing is on a PC running Firefox browser. Try ahead of time as you may have to download a ‘plugin’.

Link for polar and nonpolar compounds: https://lab.concord.org/embeddable.html#interactives/interactions/elements-and-polarity.json (look at the surface plot of the molecule, but beware, the legend in the lower left corner is not correct for assigning charge to color. Actually red is negative and blue is positive charge.
Also look at Models 360 to view some surface models of other polar and nonpolar molecules.

Polar and nonpolar molecules: https://lab.concord.org/embeddable.html#interactives/interactions/boiling-point-polar-nonpolar.json

Explore attractive interactions between polar and nonpolar molecules: https://lab.concord.org/embeddable.html#interactives/interactions/comparing-polar-non-polar.json

Hydrogen bonding model of water: http://lab.concord.org/embeddable.html#interactives/sam/intermolecular-attractions/6-hydrogen-bonds-a-special-type-of-attraction.json (checkout the slow motion option)

London dispersion forces movie: http://genchem1.chem.okstate.edu/1515SP17/Personal/LDF.html

Blank DCI on IMAF: http://genchem1.chem.okstate.edu/1515SP17/Personal/IntermolecularAF.pdf

Answers: http://genchem1.chem.okstate.edu/1515SP17/Personal/IntermolecularAFAns.pdf

 

 

Question

Response

What is similar and what is different about the trend in boiling points for the Group VII hydrides, compared to Group IV and V and the noble gases?

Students should indicate that the boiling point of the compounds increases with increasing number of electrons just like Group IV and Group V and the noble gases. However, the hydride with the fewest number of electrons, HF is similar to NH3, as it has the highest boiling point in the group.

Ask if anyone remembers the polarity of this compounds?

They are all polar.

Can a student summarize the observations on the trend in boiling points for the four different groups of compounds?

So students should recognize two different patterns. First, the trend in boiling points for the substances with the higher number of electrons is the same in all four groups. Second, there is a difference in the trend in the boiling point for the member with the least number of electrons in the four groups. For polar substances the boiling point of the hydride with the least number of electrons is higher compared to the other members, but when the compound/substance is nonpolar the boiling point of the member with the least number of electrons

 

Question

Response

What is similar and what is different about the trend in boiling points for the Group V hydrides, compared to Group IV hydrides and the noble gases?

Students should indicate that the boiling point of the compounds increases with increasing number of electrons just like Group IV and the noble gases. However, the hydride with the fewest number of electrons, NH3, has the highest, or nearly the highest boiling point in the group.

Ask if anyone remembers the polarity of the members of the Group V hydrides ?

They are all polar.

 

Another approach for discussing the boiling points for Group V compounds would be to show Figure IIIA.

 

Figure III : Adding the boiling point for the Group V hydrides (without the boiling point of NH3)

 

Question

Response

Now before asking the questions associated with Figure III, begin with the question asking students to predict the boiling point of NH3.

Students should predict -110 C or something close. They will make this prediction becasue they are following a pattern that was observed in the Group IV hydrides and the Group VIII elements.

So this is the first introduction to something new, some kind of new interaction that is occuring between NH3 molecules.

Clearly, since the boiling point for NH3 is considerably higher compared to the other members there must be something going on between the molecules of NH3 that is different compared to what is going on between the molecules of the other Group V compounds.

At this point it may be worthwhile to discuss what happens when the liquid phase of a pure substance changes to the gas phase. A general chemical equation describing this change would be;

A(l) --> A(g)

The point of bring up the chemical equation of the phase change is to get the students to begin thinking about what must happen for particles of A, in the liquid phase to

 

Question

Response

What is similar and what is different about the trend in boiling points for the Group VII hydrides, compared to Group IV and V and the noble gases?

Students should indicate that the boiling point of the compounds increases with increasing number of electrons just like Group IV and Group V and the noble gases. However, the hydride with the fewest number of electrons, HF is similar to NH3, as it has the highest boiling point in the group.

Ask if anyone remembers the polarity of this compounds?

They are all polar.

Can a student summarize the observations on the trend in boiling points for the four different groups of compounds?

So students should recognize two different patterns. First, the trend in boiling points for the substances with the higher number of electrons is the same in all four groups. Second, there is a difference in the trend in the boiling point for the member with the least number of electrons in the four groups. For polar substances the boiling point of the hydride with the least number of electrons is higher compared to the other members, but when the compound/substance is nonpolar the boiling point of the member with the least number of electrons