I will just post the activity, therefore you can go about teaching the lesson any way you would like. First you will need two different colored marshmellows. With the marshmellows you can have your students create compounds. While creating their marshmellow compounds the students will learn a variety of new vocaulary.
This lesson may work out best if you choose one type of compound and have each student build it. It will cut down on confusion. For instance I had my class make a water compound with two hydrogen marshmellows and one oxygen marshmellow. It was a blast and the kids loved eating their marshmellows at the end.
If you make a worksheet for at the end and grade it you can be sure that your students learned from the actvity.
Breif: Have your students create compounds out of "molecules" of play dough and toothpicks.