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Question 4.7: Assigning Oxidation Numbers Use the rules from Table 4.5 to ...

Assigning Oxidation Numbers Use the rules from Table 4.5 to obtain the oxidation number of the chlorine atom in each of the following: (a) HClO_4 (perchloric acid), (b) ClO_3^- (chlorate ion).

PROBLEM STRATEGY

In each case, write the expression for the sum of the oxidation numbers, equating this to zero for a compound or to the charge for an ion (Rule 6). Now, use Rules 2 to 5 to substitute oxidation numbers for particular atoms, such as -2 for oxygen and +1 for hydrogen, and solve for the unknown oxidation number (Cl in this example).

Table 4.5

Rules for Assigning Oxidation Numbers
Rule Applies to Statement
1 Elements The oxidation number of an atom in an element is zero.
2 Monatomic ions The oxidation number of an atom in a monatomic ion equals the charge on the ion.
3 Oxygen The oxidation number of oxygen is  -2 in most of its compounds. (An exception is O in H_2O_2 and other
4 Hydrogen The oxidation number of hydrogen is + 1 in most of its compounds. (The oxidation number of hydrogen is -1 in binary compounds with a metal, such as CaH_2.)
5 Halogens The oxidation number of fluorine is  -1 in all of its compounds. Each of the other halogens (Cl, Br, I) has an oxidation number of  -1 in binary compounds, except when the other element is another halogen above it in the periodic table or the other element is oxygen.
6 Compounds and ions The sum of the oxidation numbers of the atoms in a compound is zero. The sum of the oxidation numbers of the atoms in a polyatomic ion equals the charge on the ion.
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