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# Question Video: Determining the Oxidation Number of Atoms in a Molecule, Ion, and Element Chemistry

What is the sum of the oxidation numbers of sulfur in the following chemical species: S₈, H₂S, SO₂, SO₃, S²⁻?

03:56

### Video Transcript

What is the sum of the oxidation numbers of sulfur in the following chemical species: S8, H2S, SO2, SO3, S2−?

This problem is asking us to identify the oxidation numbers within each chemical species and then add the oxidation numbers together. Before we begin to solve this problem, let us look at oxidation numbers. Oxidation numbers show the degree of oxidation of an atom in isolation or in a compound in terms of counting electrons. To help us, there are rules for oxidation numbers. Let us look at these rules and apply them to each chemical species within the problem.

The first rule we will discuss is oxidation numbers of an atom or molecule in its elemental form is zero. This rule applies to S8. The oxidation number of S8 is zero since it is in elemental form. The next rule we will apply is about oxygen. The oxidation number for oxygen in a compound is negative two. Let us take a look at SO2. Since each oxygen has a charge of negative two and there are two oxygen atoms, the overall charge of oxygen is minus four within SO2. To identify the oxidation number of sulfur, we need to note that the oxidation numbers in a neutral compound is zero. The molecule SO2 needs to have a net charge of zero. Therefore, the charge on the one sulfur in SO2 must have the charge of positive four to balance the charge of negative four from oxygen.

Now, let us identify the oxidation number of sulfur in SO3. We just reviewed that the oxidation number for oxygen in a compound is negative two. Therefore, each oxygen has a charge of negative two, and there are three oxygen atoms. Therefore, the overall charge of oxygen is negative six. The compound SO3 needs to have a net charge of zero. Therefore, the charge on the one sulfur in SO3 must have the charge of positive six to balance out the charge of negative six from the three oxygen ions.

Now, let us look at the rule for hydrogen to identify the oxidation number of sulfur in H2S. The oxidation number for hydrogen in a compound is positive one. When looking at H2S, there are two hydrogen ions, each with a charge of positive one. The overall charge of hydrogen is plus two. The compound H2S needs to have a net charge of zero. Therefore, the charge on the one sulfur in H2S must have a charge of negative two.

The last rule we will discuss pertains to ions. An oxidation number for an ion is the same as the charge of the ion. When looking at S2−, since the charge of the sulfide ion is two negative, the oxidation number is negative two.

Now that we know the oxidation numbers of sulfur in each chemical species — S8 is zero, SO2 is positive four, SO3 is positive six, H2S is negative two, and S2− is negative two — we can add the oxidation numbers together to calculate a sum of six. Therefore, the sum of the oxidation numbers of sulfur in the chemical species is six.

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