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When my high school chemistry class gets to polar molecules,听we run into trouble because most of the students have never听heard of the word 鈥渧ector鈥� and, even though we鈥檝e covered听VSEPR theory, they don鈥檛 have the spatial reasoning abilities to听see how polar bonds could cancel out or contribute to a dipole听by looking at drawings on the board. After doing my traditional听explanation of how polar bonds can create polar molecules听using Lewis diagrams and arrows, we move to the lab portion of the classroom where we have more space. I ask a couple of volunteers to model a simple molecule like HCl using a long听(1-2 m) piece of tubing.

The rules for the molecules are as follows:

• The tubing represents the bonding electrons.
• Students represent the atoms.
• 3D shapes of molecules are determined by VSEPR theory.
• Students tug on the tubing based on the electronegativity听of the atoms they represent.
• Lone pairs are not depicted.

Since the tubing is stretchy and can be pulled by the students,听we have the ability to depict polar bonds and thus, polar听molecules. With help from the spectators, we figure out the听effect the electronegativity of chlorine and hydrogen have on听how hard each person tugs on the tubing. I note that hydrogen can get only so close to chlorine due to repulsion of their
electron clouds, so the hydrogen person should not be moved听by the tugging chlorine, just leaning toward the chlorine person.听Then I ask the audience if this is a polar molecule, and they听usually all get that it is, indeed, polar. To make sure everyone听thoroughly understands, I give an explanation, sometimes听referring to a Lewis structure drawn on a nearby white board. It鈥檚听usually helpful to ask the volunteers to turn themselves into a听diatomic molecule such as H₂ or F₂ to show that no matter how hard the atoms tug on the electrons in this tug-of-war match, the key is how hard they pull relative to their competition.听

Next, we model CO₂, a more complex situation. I ask for three听volunteers and give them four pieces of tubing, noting that two听pieces of tubing represent a double bond. As they get听themselves into position, often with the help of a Lewis structure,听I review VSEPR theory by reminding everyone that electrons听position themselves so that they are as far apart as possible.听

The audience critiques the model until all three people are听positioned in a straight line a couple meters long. Next, we听factor in the electronegativity of each atom so the oxygen people听start pulling on the central carbon person (the oxygens might听need to be reminded that they are essentially twins with equal听electronegativity/strength). I ask the carbon person if she is听being pulled one way or another. After some thought, she usually concludes 鈥渘o鈥�. I explain to the class that this is a听situation in which polar bonds cancel each other out and refer to听a Lewis structure of CO2 showing two dipole arrows.听Said in simpler terms, since the carbon person is not being forced to听lean in one direction or another, this molecule is nonpolar.

Moving on to an even more complex situation, I ask for four听volunteers, give them three pieces of tubing and ask them to听model BF₃, a trigonal听planar molecule. It鈥檚 a little tricky to get all听the bond angles to be 120掳 and for everyone to pull equally, but听with a stretch of the imagination, everyone, especially the听central boron person, can tell that boron鈥檚 electrons are not听being pulled in any one direction and that this molecule is听nonpolar. This is easier to accept if I give this same group听 another piece of tubing and tell them to model CH2O, another trigonal planar molecule. As the volunteers consider their听electronegativity and adjust their tugging strength, everyone,听including the audience, can see that the central carbon is pulled听in one direction, toward the oxygen, making this variation of a听trigonal planar molecule polar.

Now that the class understands how to model molecules and has warmed up with simple linear or planar particles, we move听on to even more challenging molecules: those with tetrahedral听and pyramidal shapes. Since we have already discussed听VSEPR theory and the impact lone pairs and bonding electrons听have on molecular geometry, we can focus on the effect that听shape has on molecule polarity.

First, I ask a group of students to model NH3, a trigonal pyramidal molecule. At first, they usually make a planar听molecule, so I remind them that the 3D shape is really important听in determining polarity. After a little thinking, often times with听suggestions from the spectators, the hydrogen students squat听down and the nitrogen student stands tall, often holding the听tubing above his head.

For the final challenge, I ask a group of volunteers to model CF₄. It takes a bit of creativity to arrange themselves in a tetrahedron,听but once the molecule is properly formed and the fluorine people听start pulling on the tubing, the carbon person realizes that there听is no clear direction in which he is being pulled. (It鈥檚 unlikely that听the carbon will feel no net tugging direction since it鈥檚 really hard听to get the angles and pulling strength perfect in a tetrahedron).听

Once everyone understands why this is a nonpolar molecule, we听go through related molecules by substituting hydrogens for听fluorines, asking the carbon to report on the net tugging direction听that he feels.

To get the most mileage out of this activity, I don鈥檛 randomly assign people to portray the atoms. Instead, I pick the people听who have a very weak understanding of polarity (or chemistry in听general) and ask those people to be the central atom so that听they can benefit the most from the first-hand, kinesthetic听experience of this activity. By repeatedly asking for a new set of听volunteers, nearly all the students are able to experience
polarity. It鈥檚 helpful to model particles whose Lewis structures we have already drawn and are displayed somewhere in the room. I also have some ball-and-stick models handy to remind听the students what these shapes look like and help position听themselves accurately.

Once we鈥檝e finished this activity, the students definitely have an improved comprehension of polarity and I find it is a memorable听activity that I can refer to later in the year when someone asks听me a question about dipoles. Not only does the activity help my听students understand polarity, but it鈥檚 also very effective at听deepening their comprehension of electronegativity, VSEPR听theory and molecular geometry, plus, it improves their ability to
look at a 2D Lewis structure and visualize what these particles听look like in 3D.听

HCl

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Water molecule听

Water molecule - pulling from the centre between two students听

Student pulling from four ends of rubber tubing with each end being pulled by a student to demonstrate the polarity of CH₄听molecule听