To predict the three-dimensional structure of a covalent molecule given its electron dot structure, I will use toothpicks and gumdrops. The toothpicks will be stuck into the gumdrops to connect together and form a molecule. Each student will get a generous amount of small gumdrops for their electrons and a handful of large gumdrops to use as the central atom. At first to get students thinking about how to make electrons spaced out the farthest apart from each other, I will have them create the basic geometric shapes such as a line, triangle, square, pentagon, and hexagon. I will then give an example of a tetrahedral arranged in the wrong order and ask the students if this is the best way to make the electrons farthest from each other. If they can identify why it is wrong I will then let the students figure out how to make each arrangement of electrons around a central atom (linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral). They will have to demonstrate an understanding of how to get all the electrons around a central atom before moving on to the more complex idea of representing a covalent molecule when given its electron dot structure. After students demonstrate an understanding of the basic arrangement of electrons around a single atom, they will have to construct a molecule with their gumdrops and toothpicks. At first, the molecules will be simple such a H2O eventually working towards molecules that are more difficult like CH3N.
There is only one safety issue with using toothpicks is that they can be sharp. Students would need to understand that in order to do this activity they must use them properly and be safe while using them. To use gumdrops and toothpicks, however, is cost efficient. A pack of 3200 toothpicks would cost me $3.28 and a five pound bag of gum drops is $9.75 plus a small amount for taxes. It is a way to get students involved with a hands on understanding of the lesson objective. Students will be able to manipulate a model molecule that is not so abstract. This tool is better than using a virtual manipulative because students can see the three-dimensional aspect rather than having to imagine what it may look like. It also forces students to construct the correct arrangement themselves. Most importantly, students can eat the gumdrops at the end of the lesson, which always makes a student happy to do the work.