Nitrogen: The Main Gas in Air

Introduction

Almost four-fifths of every breath is nitrogen, yet your body does nothing with it: the nitrogen you breathe in comes straight back out. Nitrogen is the most abundant gas in air and one of the least reactive. This page explains what nitrogen is like, why it is so reluctant to react, and why living things still depend completely on nitrogen once it has been locked into compounds.

Core explanation

Abundance. Nitrogen makes up about 78% of dry air by volume. It exists as diatomic molecules, N₂, in which two nitrogen atoms are joined together.

Physical properties. Nitrogen is colourless, odourless and tasteless. It is only very slightly soluble in water and has a density a little less than that of air. It boils at about −196 °C, so it is a gas at all everyday temperatures.

Chemical properties. Nitrogen does not burn, does not support burning and does not turn limewater milky. A lighted splint put into nitrogen simply goes out. Most substances do not react with it at room temperature.

Why so unreactive? The two atoms in an N₂ molecule share three pairs of electrons, forming a triple bond . This is one of the strongest bonds between two atoms in chemistry. Before nitrogen can react, this bond must be broken, and that needs a great deal of energy. At everyday temperatures, collisions between molecules do not supply enough energy, so nitrogen stays unchanged.

When nitrogen does react. At very high temperatures the triple bond can be broken. Inside a car engine, or in a lightning flash, some nitrogen combines with oxygen to form nitrogen oxides (such as NO and NO₂), which are air pollutants. In industry, nitrogen is combined with hydrogen at high temperature and pressure, with a catalyst, to make ammonia (NH₃) for fertilisers.

Nitrogen and life. Proteins and DNA contain nitrogen, but plants and animals cannot use N₂ directly. They rely on nitrogen fixation : certain bacteria, lightning and the fertiliser industry convert nitrogen into compounds such as nitrates and ammonium salts, which plants absorb through their roots. Animals get their nitrogen by eating plants or other animals.

Formulae

Nitrogen molecule: N₂ (N≡N, a triple bond). Word equation at high temperature: nitrogen + oxygen → nitrogen monoxide; N₂ + O₂ → 2NO.

Step-by-step reasoning

Why can a gas making up 78% of air be so unreactive?

1. Every nitrogen molecule contains a triple bond. 2. Breaking this bond requires a very large amount of energy. 3. Room-temperature collisions supply much less energy than this. 4. So nitrogen molecules bounce off other molecules without reacting.

Visual explanation

Draw two nitrogen atoms side by side with three lines between them, N≡N. Compare this with oxygen, O=O, which has only two lines. The extra line is a picture of the extra shared pair that makes nitrogen so hard to pull apart.

Real-world analogy

A nitrogen molecule is like two people holding hands with three tight grips at once. Someone trying to separate them with a gentle push gets nowhere; only a very forceful effort, like a lightning strike, can break them apart.

Real-world example

Crisp packets are filled with nitrogen rather than air. Because nitrogen does not react with the food, the crisps do not go stale or rancid as quickly, and the cushion of gas stops them being crushed.

Why?

Why does most of the nitrogen we breathe come back out unchanged? Our bodies have no way of breaking the triple bond. The nitrogen dissolves slightly in the blood and lungs but takes no part in respiration, so it is simply exhaled.

Common misconception

"Nitrogen is useless because it is unreactive." In fact its unreactivity is exactly what makes it useful: it protects food, prevents fires in fuel tanks and dilutes the oxygen in air so that burning is not too fierce.

Worked example

Question: A room contains 50 m³ of air. Estimate the volume of nitrogen present and explain why the amount barely changes when people breathe in the room for an hour.

Reasoning: 78% of 50 m³ = 0.78 × 50 = 39 m³. Respiration uses oxygen and produces carbon dioxide and water; nitrogen does not take part.

Answer: About 39 m³; the nitrogen is breathed in and out unchanged.

Quick check

1. What type of bond holds the two atoms together in a nitrogen molecule? Answer: A triple covalent bond (three shared pairs of electrons).

Exam focus

Be ready to state that nitrogen is about 78% of air, is unreactive because of its strong triple bond, and reacts with oxygen only at high temperatures such as in engines or lightning. Link this to the formation of nitrogen oxide pollutants.

Advanced insight

The energy needed to break one mole of N≡N bonds is about 945 kJ, compared with about 498 kJ for the O=O bond. Nitrogen-fixing bacteria break this bond at ordinary temperatures using an enzyme called nitrogenase, something industry can only achieve with high temperatures, high pressures and a catalyst.

Summary

Nitrogen, N₂, is a colourless, odourless gas that makes up about 78% of air. It is very unreactive because its atoms are held by a strong triple bond. It reacts only under extreme conditions, forming nitrogen oxides or ammonia. Living things need nitrogen in fixed forms such as nitrates.

Practice questions

1. Give two physical properties of nitrogen. Answer: It is colourless and odourless (also tasteless, only slightly soluble in water, boils at about −196 °C). 2. Explain why nitrogen is used to fill food packets. Answer: It is unreactive, so it does not react with the food, helping it stay fresh for longer. 3. Name one situation in which nitrogen reacts with oxygen and name the type of product formed. Answer: In a car engine or in lightning; nitrogen oxides are formed. 4. Why can plants not use the nitrogen in air directly? Answer: They cannot break the strong triple bond in N₂; they need nitrogen in fixed compounds such as nitrates.