Answer:+6
Explanation:Oxygen always has an ON of -2 so when there’s O7 the total is -12. Sodium is in group 1 so has an ON of +1 that means with two of them it’s overall ON is +2. When you combine the two that equals -12 but you want it to equal 0. So it would be +12. Considering there are two chromium present you would divide that by 2 making the answer +6.