An oxidation number represents a theoretical charge assigned to an atom within a compound or ion. It helps chemists track electron distribution and analyze chemical reactions, particularly those involving oxidation and reduction processes. This value is a tool, not a true physical charge, and can vary for an element based on its chemical environment.
Fundamental Rules for Oxidation Numbers
Assigning oxidation numbers relies on a set of established rules that prioritize certain elements and bonding scenarios. An uncombined element in its free state, such as elemental oxygen (O₂) or sodium metal (Na), always has an oxidation number of zero. For a monatomic ion, its oxidation number is equivalent to that charge; for instance, a sodium ion (Na⁺) has an oxidation number of +1, and a chloride ion (Cl⁻) has an oxidation number of -1.
When atoms combine to form a neutral compound, the sum of all their individual oxidation numbers must equal zero. If the atoms form a polyatomic ion, the sum of their oxidation numbers must instead equal the overall charge of that ion. Common oxidation numbers for specific elements include:
- Oxygen typically has an oxidation number of -2 in most compounds, though exceptions exist in peroxides where it is -1, or when bonded to fluorine, where it can be positive.
- Hydrogen usually exhibits an oxidation number of +1, except in metal hydrides, where it is -1.
- Group 1 metals consistently show an oxidation number of +1.
- Group 2 metals are always +2 in compounds.
- Fluorine, being the most electronegative element, consistently has an oxidation number of -1 in all its compounds.
Chlorine’s Oxidation Numbers in Various Compounds
Chlorine can display a wide range of oxidation numbers, from -1 to +7, depending on the elements it is bonded with, especially when oxygen is present. In its elemental form, such as chlorine gas (Cl₂), chlorine has an oxidation number of 0. When chlorine forms a simple chloride ion (Cl⁻), its oxidation number is -1. In compounds like hydrogen chloride (HCl), hydrogen typically has an oxidation number of +1, meaning chlorine’s oxidation number is -1 (+1 + (-1) = 0).
Chlorine exhibits positive oxidation numbers when bonded to more electronegative elements like oxygen. In the hypochlorite ion (ClO⁻), oxygen typically has an oxidation number of -2. Since the overall charge of the ion is -1, the oxidation number of chlorine (Cl) can be calculated as: Cl + (-2) = -1, which results in chlorine having an oxidation number of +1. Moving to the chlorite ion (ClO₂⁻), with two oxygen atoms each at -2, the total contribution from oxygen is -4. For the ion’s overall charge of -1, chlorine’s oxidation number must be +3 (Cl + 2(-2) = -1, so Cl – 4 = -1, leading to Cl = +3).
In the chlorate ion (ClO₃⁻), there are three oxygen atoms, contributing a total of -6 to the oxidation state. To achieve the ion’s -1 charge, chlorine’s oxidation number is +5 (Cl + 3(-2) = -1, so Cl – 6 = -1, resulting in Cl = +5).
The highest common oxidation number for chlorine is observed in the perchlorate ion (ClO₄⁻). With four oxygen atoms, their combined oxidation number is -8. For the overall ion charge of -1, chlorine must have an oxidation number of +7 (Cl + 4(-2) = -1, so Cl – 8 = -1, giving Cl = +7). This progression demonstrates how chlorine’s oxidation number increases as it bonds with more oxygen atoms.