When elements in column seven of the periodic table (e.g., F…

When elements in column seven of the periodic table (e.g., F, Cl, Br) become ions, why do they become ions with -1 charge?  (Write no more than 2-3 sentences.)  (Hint: Think octet rule.)  (Note: You must adequately explain/defend your answer to demonstrate that you truly understand the material.  Extra time has been provided to allow you to write an answer to this question.)