­In this activity, students self-organize according to periodic trends. I print out the attached cards onto card stock (each page will contain two) and hand them out to the students (one to each). Generally, we go outside and I shout out periodic trends (i.e. size, polarizability, ionization energy, Z_eff etc.) and the students run to get in line in the correct order.  I have a bell which I ding if correct, and a buzzer that I sound if incorrect. If incorrect, they have to try again.