An oxidation number, often called an oxidation state, is a hypothetical charge assigned to an atom within a molecule or ion. This assignment assumes all bonds are purely ionic, with electrons completely transferred to the more electronegative atom. The oxidation number is a tool used by chemists to track electron distribution in chemical species. Its primary purpose is to identify which atoms are gaining (reduction) or losing (oxidation) electrons during redox reactions.
Fundamental Rules for Assigning Oxidation Numbers
For simple inorganic compounds, a set of foundational rules determines the oxidation number of an element. The oxidation number of a pure element in its elemental state, such as O₂ or C (graphite), is always zero. For monatomic ions, the oxidation number equals the ion’s charge, such as +1 for Na⁺.
Within a compound, certain elements have consistent assignments: Group 1 metals are +1, Group 2 metals are +2, and fluorine is always -1. Oxygen is usually -2, and hydrogen is typically +1 when bonded to a nonmetal.
The sum of all oxidation numbers in a neutral compound must equal zero, and in a polyatomic ion, the sum must equal the charge of the ion. These general rules, however, are often insufficient when dealing with the complexity of organic molecules containing carbon.
The Unique Variability of Carbon’s Oxidation States
Carbon’s oxidation number does not have a single answer due to its unique chemical properties. Carbon is tetravalent, meaning it forms four covalent bonds with other atoms, including other carbon atoms.
Its intermediate electronegativity allows it to form stable bonds with elements that are both more electronegative (like oxygen and nitrogen) and less electronegative (like hydrogen). This versatile bonding allows carbon to exist in compounds with oxidation states ranging from its most reduced state of -4 to its most oxidized state of +4.
Carbon’s ability to form long chains, complex rings, and various functional groups means that two carbon atoms in the same molecule can possess different oxidation numbers. This wide variability necessitates a structure-based method for accurate determination, particularly in organic chemistry.
Structure-Based Method for Calculating Carbon’s Oxidation State
Determining the oxidation state of carbon in complex organic molecules requires a specialized method focused on its individual bonds, rather than relying on overall charge balance. This approach treats each bond to the carbon atom of interest as if the shared electrons were completely transferred to the more electronegative partner.
The initial step involves drawing the full Lewis structure to clearly show all bonds connected to the specific carbon. Next, a hypothetical charge is assigned to the carbon for each bond based on the electronegativity difference with the partner atom.
For every bond between carbon and a less electronegative atom (like hydrogen), the carbon is assigned an oxidation number contribution of -1. Conversely, for every bond to a more electronegative atom (like oxygen or nitrogen), the carbon is assigned an oxidation number contribution of +1. Bonds between two carbon atoms are ignored entirely, as there is no electronegativity difference.
Finally, the oxidation state for that specific carbon atom is found by summing the contributions from all four of its bonds. If the carbon is double- or triple-bonded, the bond contributes -2 or -3 (less electronegative partner) or +2 or +3 (more electronegative partner) to the total, as each shared electron pair is counted separately. This atom-by-atom breakdown provides the necessary precision for organic chemistry.
Common Examples of Carbon Oxidation Numbers
The structure-based calculation method demonstrates the full range of carbon’s oxidation states across common compounds. In methane (CH₄), carbon is bonded to four less electronegative hydrogen atoms. Each C-H bond contributes -1, resulting in the most reduced oxidation state of -4.
In methanol (CH₃OH), one hydrogen is replaced by a more electronegative oxygen atom. The three C-H bonds contribute -3, and the single C-O bond contributes +1, giving carbon an oxidation state of -2.
In formaldehyde (CH₂O), the carbon atom is double-bonded to one oxygen and single-bonded to two hydrogen atoms. The two C-H bonds contribute -2, while the C=O double bond contributes +2, leading to a net oxidation state of 0. The maximum oxidized state is seen in carbon dioxide (CO₂), where the carbon is double-bonded to two oxygen atoms. Each C=O bond contributes +2, summing to the most oxidized state of +4.