Question

What is the oxidation number of chlorine in $ClO_3^ -$?
(A) $+ 5$
(B) $+ 3$
(C) $+ 4$
(D) $+ 2$

Answer 1

Oxidation number is defined as the charge which an atom of an element has in its ion or appears to have when present in the combined state with other atoms .

Complete step by step answer:
Calculating oxidation numbers:
An oxidation number can be assigned to a given element or compound by following rules:
1. Any free element has an oxidation number equal to zero.
2. For monatomic ions, the oxidation number always has the same value as the net charge corresponding to the ion.
3. The hydrogen atom exhibits an oxidation state of $+ 1$ . However when bonded with an element with less electronegativity than it, it exhibits an oxidation number of $- 1$
4. Oxygen has an oxidation of $- 2$ in most of its compounds. However, in the case of peroxides the oxidation number corresponding to oxygen is $- 1$.
5. All alkali metals (group $1$ elements) have an oxidation state of $+ 1$ in their compounds.
6. All alkaline earth metals (group $2$ elements) exhibit an oxidation state of $+ 2$ in their compounds
7. In the compounds made up of two elements, a halogen (group $17$ elements) have an oxidation number of $- 1$ assigned to them.
8. In case of neutral compounds, the sum of all the oxidation numbers of the constituent atoms totals zero.
9. When polyatomic ions are considered the sum of all oxidation numbers of the atoms that constitute them equals the net charge of the polyatomic ion.
Thus, the oxidation number of an atom in a given compound can be calculated with the steps mentioned above:
$ClO_3^ -$
Let $x$ be the oxidation number of chlorine in $ClO_3^ -$.
The oxidation number of oxygen is $- 2$ .
The sum of the oxidation number of chlorine and oxygen is, $- 1$ , which is equal to the charge on ions.
Hence,
$x + 3\left( { - 2} \right) = - 1$
$x = + 5$
So the correct answer is A.

Note:
Oxidation states can be useful in working out of stoichiometry for titration reactions when there is insufficient information to work out the complete ionic equation. Each time an oxidation state changes by one unit, one electron has been transferred. If the oxidation state of one substance in a reaction decreases by $2$ , it has gained by $2$ electrons.