Question

The oxidation number of chlorine in $NaCl{O}_3$ is:

• A. $-5$
• B. $+5$
• C. $+2$
• D. $-3$

Answer 1

Answer: (B)

The overall charge of molecule is zero (because it's not an ion). Since sodium is a 1A family member, you can assume that the charge is $+1$. The charge of oxygen is almost always $-2$ so you can assume that as well.
$(+1)+$ oxidation of $Cl+3(-2)=0$
So the oxidation of chlorine in that problem is $+5$.

So answer is C.