I need help making a model of a krypton atom? I'm not sure what to make it out of?

Ideally you should have 36 small balls for the protons rather than one large one. Protons are larger than electrons, so you need another 36 which are smaller still. If you visit a craft shop you should be able to find suitable balls in wood or foam. Glue the protons together into a ball and then drill fine holes for lengths of piano wire to suspend the electrons.