Question

Assign oxidation numbers to each element in (a) methane (b) carbonate ion \(\left(\mathrm{CO}_{3}^{2-}\right)\) (c) iodate ion \(\left(\mathrm{IO}_{4}^{-}\right)\) (d) hydrazine \(\left(\mathrm{N}_{2} \mathrm{H}_{4}\right)\)

Short answer

Answer: (a) Methane (CH4): C: -4, H: +1 (b) Carbonate Ion (CO3^2-): C: +4, O: -2 (c) Iodate Ion (IO4^-): I: +7, O: -2 (d) Hydrazine (N2H4): N: -2, H: +1

Step-by-step solution

Rule Summary

Before assigning oxidation numbers, let's quickly summarize the rules we need to follow: 1. The oxidation number of an atom in an element is always 0. 2. The oxidation number of a monoatomic ion is equal to its charge. 3. Oxygen usually has an oxidation number of -2 in compounds. 4. Hydrogen usually has an oxidation number of +1 in compounds. 5. The sum of the oxidation numbers in a neutral compound is 0. 6. The sum of the oxidation numbers in a polyatomic ion is equal to its charge. Now let's apply these rules to find the oxidation numbers for each element in the given compounds.

(a) Methane (CH4)

Since hydrogen usually has an oxidation number of +1, and methane is a neutral compound, the sum of the oxidation numbers must equal 0. We have 4 hydrogens, so the oxidation number of carbon must be -4 (since +4 from hydrogen atoms and -4 from carbon atoms result in zero overall). The oxidation numbers are: C: -4 H: +1

(b) Carbonate Ion (CO3^2-)

Oxygen usually has an oxidation number of -2. We have 3 oxygen atoms, which would contribute a total of -6. For the sum of the oxidation numbers to equal the charge of the ion (-2), carbon must have an oxidation number of +4. The oxidation numbers are: C: +4 O: -2

(c) Iodate Ion (IO4^-)

Again, oxygen usually has an oxidation number of -2. With 4 oxygen atoms, this contributes a total of -8. For the sum of the oxidation numbers to equal the charge of the ion (-1), iodine must have an oxidation number of +7. The oxidation numbers are: I: +7 O: -2

(d) Hydrazine (N2H4)

In hydrazine, hydrogen usually has an oxidation number of +1. We have 4 hydrogen atoms, so that contributes a total of +4. We also have 2 nitrogen atoms, and they have the same oxidation state in this molecule. Since hydrazine is a neutral compound, the sum of the oxidation numbers must equal 0, and so each nitrogen atom must have an oxidation number of -2. The oxidation numbers are: N: -2 H: +1