Using oxidation states? Seems an odd way to go about it (I tend to just balance the number of atoms), but if you look at the o.s. of the chlorine's on both sides as it's the only thing where the o.s. has changed.
and i thought about that but the chlorine has oxidation states ( if i worked it our right) then it has oxidation states of +5 and -1
So one chlorine atom gained an electron, the other lost 5. One of the five lost went to what is now a chloride ion. So what do you think happened to the other 4 electrons?