The oxidation number of sulfur (S) in sulfur dioxide (SO2) is +4.
Understanding how to determine oxidation numbers is fundamental in chemistry, especially for analyzing redox reactions and naming compounds. The process for finding the oxidation number of an atom in a compound like SO2 involves applying a set of established rules.
What is an Oxidation Number?
An oxidation number (or oxidation state) represents the hypothetical charge an atom would have if all bonds were purely ionic. It's a way to track electron distribution in compounds and is crucial for identifying which atoms are oxidized (lose electrons) or reduced (gain electrons) in a chemical reaction.
Step-by-Step Calculation for SO2
To find the oxidation number of sulfur in SO2, we follow these steps:
- Identify the known oxidation numbers:
- Oxygen nearly always has an oxidation number of -2 in compounds (exceptions include peroxides like H2O2 where it's -1, and superoxides where it's -1/2). In SO2, oxygen is in its typical state.
- Set up the equation:
- The sum of the oxidation numbers of all atoms in a neutral compound must equal zero.
- Let 'x' be the oxidation number of sulfur.
- There is one sulfur atom and two oxygen atoms.
- The equation becomes: (1 × Oxidation Number of S) + (2 × Oxidation Number of O) = 0
- Substituting the known value for oxygen: (1 × x) + (2 × -2) = 0
- Solve for the unknown:
- x + (-4) = 0
- x - 4 = 0
- x = +4
Therefore, the oxidation number for sulfur in SO2 is +4. This means the two oxygen atoms collectively contribute a total of -4 to the overall oxidation state, and to balance this out and achieve a neutral compound, the sulfur atom must have a positive 4 oxidation number.
Summary Table for SO2
| Element | Number of Atoms | Typical Oxidation Number | Total Contribution |
|---|---|---|---|
| Sulfur | 1 | x | +4 |
| Oxygen | 2 | -2 | -4 |
| Total | 0 |
General Rules for Assigning Oxidation Numbers
To accurately determine oxidation numbers, keep these general rules in mind:
- Elemental Form: An atom in its elemental form (e.g., S, O2, Na, Cl2) has an oxidation number of 0.
- Monatomic Ions: The oxidation number of a monatomic ion (e.g., Na+, Cl-, O2-) is equal to its charge.
- Group 1 Metals: Alkali metals (Li, Na, K, etc.) always have an oxidation number of +1 in compounds.
- Group 2 Metals: Alkaline earth metals (Be, Mg, Ca, etc.) always have an oxidation number of +2 in compounds.
- Hydrogen: Hydrogen usually has an oxidation number of +1 when bonded to nonmetals and -1 when bonded to metals (e.g., NaH).
- Oxygen: Oxygen usually has an oxidation number of -2 (as seen in SO2). Exceptions include peroxides (e.g., H2O2) where it's -1, and superoxides where it's -1/2.
- Halogens: Fluorine always has an oxidation number of -1. Other halogens (Cl, Br, I) usually have -1 unless they are bonded to oxygen or a more electronegative halogen.
- Sum in Compounds: The sum of all oxidation numbers in a neutral compound is zero.
- Sum in Polyatomic Ions: The sum of all oxidation numbers in a polyatomic ion (e.g., SO4^2-, NO3-) is equal to the charge of the ion.
For more detailed information on oxidation states and how they apply in various chemical contexts, you can explore resources like Khan Academy's lessons on oxidation states or LibreTexts Chemistry on oxidation numbers.
Oxidation Numbers
