I used JCE Classroom Activity #111 in my chemistry classes today. (Subscription to JCE required) What a great way to help students make the connection between number of ions present, the charges of the ions and the neutral compound formula. It is easy for the teacher to spot misconceptions and address them as you move around the room and watch students complete the activity. Be careful to make sure the students are really building. I tried to make sure to see each student build at least one compound. I brought the class together for a final discussion of the activity before they completed the questions at the end of the activity and chose students to build compounds that I wrote on the board. You could use the lego sets, but I used the LEGO Digital Designer software (http://ldd.lego.com//) suggested in the lab. I didn't have my students use it for the main activity, but it would certainly be an option if your students have internet access in your class or if you wanted to send this activity home as homework.