Nitrogen, the element
Nitrogen is element 7 in group 15 (old name VA) of the periodic table. Its atom has 5 outer electrons. Two atoms share three pairs of electrons and make the N₂ molecule with a triple bond (N≡N).
- Colourless, odourless gas; 78% of air by volume.
- Very unreactive at room temperature because the triple bond needs a lot of energy to break.
- Used to fill food packets (stops food going stale), as liquid nitrogen (−196 °C) for freezing, and to make ammonia.
Phosphorus is in the same group, just below nitrogen. It is a solid, much more reactive (white phosphorus catches fire in air, so it is kept under water), and is also a key plant nutrient (in phosphate fertilisers, bones, DNA).
Nitrogen fixation and the Haber process
Nitrogen fixation means turning N₂ gas into compounds that plants can use, such as ammonia (NH₃) or nitrate (NO₃⁻). It happens in three ways:
- Biological: Rhizobium bacteria in the root nodules of pea-family plants, and some free-living bacteria and cyanobacteria.
- Lightning: the heat joins N₂ and O₂ into nitrogen oxides, which rain carries into the soil.
- Industrial (Haber process): N₂ + 3H₂ ⇌ 2NH₃, at about 450 °C, about 200 atm, with an iron catalyst.
Why these conditions?
The reaction is reversible and gives out heat. Four gas molecules become two, so high pressure pushes it towards ammonia. A low temperature would give more ammonia but far too slowly, so 450 °C is a compromise. The iron catalyst makes it faster. Unused N₂ and H₂ are cooled, ammonia is removed as a liquid, and the rest is sent round again.
Compounds of nitrogen: acids, nitrides and fertilisers
- Ammonia, NH₃: a gas with a sharp smell, very soluble in water, forms a weak alkali (turns red litmus blue). With acids it makes ammonium salts.
- Nitric acid, HNO₃: made from ammonia (Ostwald process: NH₃ is burned over a platinum catalyst to NO, then NO₂, then absorbed in water). A strong acid and a strong oxidising agent; used for fertilisers, dyes and explosives.
- Nitrous acid, HNO₂: a weak, unstable acid; its salts (nitrites) are used to preserve meat.
- Nitrides: compounds of nitrogen with a metal, e.g. 3Mg + N₂ → Mg₃N₂ when magnesium burns in air. Magnesium nitride reacts with water to give ammonia.
- Fertilisers: urea CO(NH₂)₂ (46% N), ammonium nitrate NH₄NO₃, ammonium sulfate, and NPK mixes with phosphorus and potassium.
Too much fertiliser
Extra nitrate washes into rivers and lakes. Algae grow wildly, die and rot, and the water loses oxygen (eutrophication). Fish die. Using the right amount at the right time saves money and water life.
The nitrogen cycle step by step
- Fixation: N₂ → ammonia/ammonium (NH₄⁺).
- Nitrification: nitrifying bacteria change NH₄⁺ → nitrite (NO₂⁻) → nitrate (NO₃⁻).
- Assimilation: plant roots absorb nitrate and build proteins, DNA and chlorophyll.
- Food chain: animals eat plants; nitrogen moves into animal proteins; waste such as urea leaves the body.
- Ammonification (decay): bacteria and fungi break down dead bodies and waste into ammonia.
- Denitrification: in wet, low-oxygen soil, denitrifying bacteria turn nitrate back into N₂ gas.
Humans change the cycle by making huge amounts of fixed nitrogen (fertilisers) and by burning fuels, which releases nitrogen oxides that cause acid rain and smog.
Try it: a bean-root hunt
Gently dig up a bean, pea or gram plant (or soak and sprout chana for two weeks in a pot). Wash the roots. Count the small pink-brown bumps: these are root nodules where bacteria fix nitrogen. Cut one open: a pink inside means the bacteria are active. Then open the 3D at the last step and pick "Fixation" to see where your nodules fit in the cycle.
Key formulas and definitions
- N₂ + 3H₂ ⇌ 2NH₃ (Haber: 450 °C, 200 atm, Fe catalyst)
- 4NH₃ + 5O₂ → 4NO + 6H₂O (first step to nitric acid)
- 3Mg + N₂ → Mg₃N₂ (a nitride)
- NH₃ + HNO₃ → NH₄NO₃ (ammonium nitrate fertiliser)
- NH₄⁺ → NO₂⁻ → NO₃⁻ (nitrification)
- NO₃⁻ → N₂ (denitrification)
Worked examples
1. How many molecules of ammonia can be made from 5 molecules of N₂ and plenty of H₂?
N₂ + 3H₂ → 2NH₃, so each N₂ gives 2 NH₃. 5 × 2 = 10 molecules of ammonia (and 15 H₂ are used).
2. Find the percentage of nitrogen in ammonium nitrate, NH₄NO₃ (N = 14, H = 1, O = 16).
Molar mass = 14 + 4 + 14 + 48 = 80 g/mol. Nitrogen = 2 × 14 = 28. % N = 28 ÷ 80 × 100 = 35%.
3. In the Haber process, why is a very high pressure used?
On the left there are 1 + 3 = 4 gas molecules; on the right only 2. High pressure favours the side with fewer gas molecules, so more ammonia forms.
4. Urea is CO(NH₂)₂. Show that it is about 46% nitrogen (C = 12, O = 16, N = 14, H = 1).
Molar mass = 12 + 16 + 2 × (14 + 2) = 60. N = 28. 28 ÷ 60 × 100 ≈ 46.7%.
Common mistakes
- Thinking plants take nitrogen straight from the air. Only fixing bacteria (often inside the plant's nodules) can do that; plants absorb nitrate or ammonium from soil.
- Mixing up nitrification (ammonium → nitrate) and denitrification (nitrate → N₂ gas).
- Saying the Haber process goes to completion. It is reversible; only about 15% turns into ammonia each pass, so the gases are recycled.
- Writing nitrogen as N in equations. Nitrogen gas is always N₂.