How does Iodine have 7 valence electrons?

If you look in the Periodic Table, Iodine is in Group 17, Period 5. The rule in Groups 13-18 is that there are 10 less valence electrons (except Helium) than the group number. Iodine has 53 protons, thus it has 53 electrons. The first energy level has 2. 53-2=51. Since 8 goes into 51 8 times, 8*6=48, and 51-48=3. Why doesn't Iodine have 3 valence electrons?