Molecular nitrogen, represented as N2, is the dominant form of nitrogen in Earth's atmosphere. Understanding the oxidation state of N2 is fundamental to grasping its chemical behavior and its role in both industrial processes and natural cycles. Because nitrogen is a diatomic molecule composed of two identical atoms, the distribution of electrons between them is perfectly symmetrical.
The Oxidation State of Elemental Nitrogen
By definition, the oxidation state of an element in its standard state is zero. This rule applies to all pure elements, whether they exist as monatomic gases like Argon or polyatomic molecules like O2 and N2. Since N2 consists of two nitrogen atoms sharing electrons equally in a triple bond, there is no net transfer or shift of electron density. Consequently, the oxidation state of nitrogen in N2 is 0.
Why the Oxidation State is Zero
The zero oxidation state arises from the nature of the covalent bond between the two nitrogen atoms. Each nitrogen atom has five valence electrons. To achieve a stable octet, they share three pairs of electrons, forming a triple bond. Because the atoms are identical, the bonding electrons are shared equally. There is no electronegativity difference to pull the electrons toward one atom, meaning neither atom is oxidized nor reduced.
Contrast with Compounds
It is helpful to contrast the zero state in N2 with the states nitrogen exhibits in compounds. When nitrogen combines with more electronegative elements like oxygen or halogens, it adopts positive oxidation states. For example, in nitric acid (HNO3), nitrogen has an oxidation state of +5. Conversely, when bonded to less electronegative elements like metals or hydrogen, nitrogen holds negative states, such as -3 in ammonia (NH3).
Chemical Reactivity and the Zero State
The zero oxidation state places nitrogen in a unique position known as the "middle of the redox scale." In this state, nitrogen is thermodynamically stable yet chemically inert under standard conditions due to the strong bond energy of the N≡N triple bond. For nitrogen to participate in reactions, such as nitrogen fixation, this bond must be broken. During this process, the oxidation state of nitrogen changes, either increasing (oxidation) or decreasing (reduction).
Significance in Industrial and Biological Processes
The fixation of nitrogen is the primary method by which the inert N2 molecule is converted into biologically usable forms. In the Haber-Bosch process, nitrogen is reduced to ammonia, changing its oxidation state from 0 to -3. Similarly, in biological nitrogen fixation, enzymes like nitrogenase perform this reduction. The reverse process, denitrification, involves the oxidation of nitrogen compounds back to N2, where the oxidation state returns to zero, completing the biogeochemical cycle.
Summary of Key Points
The oxidation state of N2 serves as a critical reference point in chemistry. It represents the elemental form of nitrogen, where the atom is neither losing nor gaining electrons. This neutrality explains the molecule's stability and inertness, while its ability to shift to various oxidation states makes it a central player in the chemistry of life and industry.
Compound | Oxidation State of Nitrogen
N2 (Elemental) | 0
NH3 (Ammonia) | -3
HNO3 (Nitric Acid) | +5
NO2 (Nitrogen Dioxide) | +4
N2O (Nitrous Oxide) | +1
