Biology  5/e   Raven/Johnson
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Molecular models of some sort are quite helpful. Many aspects of chemistry just don’t work on a two-dimensional surface. Use students and an object to illustrate the difference between ionic and covalent bonds. When the object is given by one student to another, the recipient can walk away, no strings attached. When the object is to be shared, analogous to the covalent bond, the two students must remain in fairly close proximity for such sharing to be practical.

In a small class setting, bring in samples of polar and nonpolar substances and mix them together. In a large class, use an overhead to project it to the entire class; this may take a little ingenuity as you will be working on a horizontal surface. Cohesion and adhesion can also be demonstrated in this manner. Petri dishes and food coloring may help. Diatec makes 35 mm deep well projection slides that are waterproof (available through Carolina or Wards Biologicals).

The following analogy has been quite helpful in differentiating ionic and covalent bonds. Mary is a well-prepared student who sits attentively in the front row during lecture. Normally she brings two cans of pop to lecture, orange and cola. Ann, a thirsty classmate, begs the cola from generous Mary and sits in the back row. The bond between the two students is analogous to an ionic bond. The can of pop is donated from one student to another. The bond strength between Mary and Ann is not very strong as they can sit on opposite sides of the lecture hall and still each drink a pop. David also comes to class with two cans of pop, root beer and lemon-lime. He, though is less generous and less decisive than Mary and wants to drink both flavors of pop during lecture. When his thirsty friend Ed arrives, David decides to share his pop rather than overtly giving one can away. Ed must, therefore, sit in the seat right next to David. This is analogous to a covalent bond. David and Ed must remain in close proximity to one another and the bond between them is quite strong, especially in comparison to the ionic bond between Mary and Ann.

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