Nitrogen

Occurrence in nature and the cycle

In nature, nitrogen is distributed extremely unevenly. Its main reservoir is the atmosphere, where it exists as the simple gas $N_2$. In the lithosphere, solid nitrogen minerals are virtually non-existent; the only major exception is the deposits of Chilean saltpetre (sodium nitrate, $NaNO_3$) in the deserts of Chile. In the biosphere, the element accumulates in biomass, as well as in peat and coal (containing 1–1.8% nitrogen).

There is a nitrogen deficiency paradox in nature. Plants are surrounded by an atmosphere consisting of 78% nitrogen, yet they often suffer from its shortage. The reason lies in the colossal strength of the $N_2$ molecule, which prevents living organisms from easily incorporating it into biochemical cycles. Upon biomass decomposition, nitrogen turns back into gas and escapes, while its soluble salts are washed out into water bodies.

Atomic structure and oxidation states

Nitrogen is a trivalent element in its ground state. Unlike many other elements, it cannot increase its valence to five by transitioning to an excited state, as its outer energy level lacks a d-sublevel.

The only way for nitrogen to become tetravalent is to give up one electron and turn into the $N^+$ cation. In this case, the s-electrons unpair, forming four unpaired electrons. This is only possible when interacting with more electronegative neighbors (fluorine or oxygen).

In the 2s-sublevel, the nitrogen atom has an unshared electron pair. It determines its properties as a donor (base) and allows it to form additional bonds via a donor-acceptor mechanism (for example, in ammonium salts or coordination complexes).

The oxidation state depends on the environment:

• In hydrazine, it is equal to –2 due to the N–N bond.
• In aminoethane — –3 (nitrogen pulls electrons away from C and H).
• In nitrobenzene — +3 (nitrogen gives 4 electrons to oxygen, but takes 1 from carbon).

Simple substance and chemical properties

The only simple substance of the element is dinitrogen ($N_2$). This is the most stable and least reactive molecule among all simple substances. It has no additional antibonding electrons like oxygen, so the bond is truly triple ($:N \equiv N:$).

Due to this strength, chemical reactions with nitrogen are difficult:

1. Under normal conditions, the gas is inert and reacts only with metallic lithium, forming a nitride on its surface: $6Li + N_2 = 2Li_3N$.
2. In an aqueous medium, there is a specific reaction of nitrogen reduction to hydrazine in the presence of $V(OH)_2$ (precipitated together with $Mg(OH)_2$): $N_2 + 4V(OH)_2 + 4H_2O = N_2H_4 + 4V(OH)_3$.
3. Upon heating to 2000 °C (or during a lightning discharge), it endothermically reacts with oxygen: $N_2 + O_2 \rightleftarrows 2NO$. The product yield is only a couple of percent.
4. With hydrogen, the reaction is exothermic. This is the basis of industrial ammonia synthesis.

Production and industrial significance

Before the early 20th century, obtaining nitrogen compounds was a most difficult task. F. Haber made a breakthrough by developing a catalyst (iron with potassium and aluminum oxides) for the direct synthesis of ammonia from hydrogen and atmospheric nitrogen.

The process takes place at a pressure of 300–500 atm and a temperature of about 300 °C. Although the product yield per cycle barely exceeds 10%, the problem is solved by recirculating the unreacted mixture. Today, this method yields over 100 million tons of ammonia per year, which goes into fertilizers and explosives.

In nature, a similar process (nitrogen fixation under mild conditions) is carried out by microorganisms — nitrifying bacteria — using the enzyme nitrogenase. Scientists have not yet managed to create an artificial analog of this enzyme to work at normal pressure and temperature.

Methods for obtaining pure $N_2$:

• In industry: low-temperature rectification of liquid air ($t_{bp} = –195.798 °C$) or chemical binding of oxygen.
• In the laboratory: mild heating of ammonium nitrite solutions ($NH_4NO_2 \rightarrow N_2 + 2H_2O$) or a mixture of $NH_4Cl$ with $NaNO_2$. Nitrogen is also released during the combustion of amines.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of action of the nitrogenase enzyme in nitrifying bacteria
• Calculation of nitrogen oxidation states in complex organic molecules
• Thermodynamics and Le Chatelier's principle in the Haber process
• Classification of nitrogen-containing compounds by oxidation state
• Donor-acceptor mechanism of formation of nitrogen coordination complexes

Other topics in "Chapter 11. Group VA Elements"

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