JEE Mains Chemistry · The p-Block Elements

Nitrogen and Its Compounds

Dinitrogen and its oxides from N₂O (+1) to N₂O₅ (+5), with the reactions that make dinitrogen, nitric oxide and nitric acid, and the tests that detect them.

All The p-Block Elements notes JEE Mains Chemistry strategy

Why this matters

Eighteen PYQs, all multiple choice, and one from 2026. Nine are about the oxides of nitrogen: oxidation states, which have an N–N bond, which are neutral and which has an odd electron; nine about making dinitrogen, the Ostwald process, the brown ring test, Nessler's reagent and the gases given off by common reactions.

Concept 1 of 2: Oxides of nitrogen: oxidation states, structures and nature

Nitrogen forms oxides in every oxidation state from +1 to +5, because it makes strong pπ–pπ bonds with oxygen. The structures follow a pattern. The two lowest oxides, N₂O and NO, are neutral. The rest are acidic. Three of them, N₂O, N₂O₃ and N₂O₄, keep an N–N bond; N₂O₅ joins its two nitrogens through an oxygen bridge instead. NO₂ has 17 valence electrons, an odd number, so one electron is unpaired on nitrogen, and two NO₂ molecules pair up through an N–N bond to give N₂O₄.

Definition

Neutral oxides: $\mathrm{N_2O}$ and $\mathrm{NO}$ . All the others are acidic.
N–N bond present: $\mathrm{N_2O}$ , $\mathrm{N_2O_3}$ , $\mathrm{N_2O_4}$ . $\mathrm{N_2O_5}$ has an N–O–N bridge and no N–N bond.
Odd electron on nitrogen: $\mathrm{NO_2}$ (and $\mathrm{NO}$ ); both are paramagnetic.
Preparations: $\mathrm{NH_4NO_3 \xrightarrow{\Delta} N_2O + 2H_2O}$ ; $\mathrm{2Pb(NO_3)_2 \xrightarrow{673\,K} 2PbO + 4NO_2 + O_2}$ ; $\mathrm{4HNO_3 + P_4O_{10} \rightarrow 2N_2O_5 + 4HPO_3}$ .
Nitrogen forms no +5 halide, because it has no d orbitals to take five bonds.

Oxide	Oxidation state of N	Structure	Nature
$\mathrm{N_2O}$	+1	Linear N≡N–O; one N–N bond	Neutral; colourless gas
$\mathrm{NO}$	+2	N=O with one unpaired electron	Neutral; colourless gas
$\mathrm{N_2O_3}$	+3	O=N–NO₂; one N–N bond	Acidic; blue solid
$\mathrm{NO_2}$	+4	Bent, odd electron on N; one N=O and one N–O	Acidic; brown gas The odd-electron oxide that dimerises to N₂O₄.
$\mathrm{N_2O_4}$	+4	O₂N–NO₂; one N–N bond, no bridging O	Acidic; colourless
$\mathrm{N_2O_5}$	+5	O₂N–O–NO₂; one N–O–N bridge, no N–N bond	Acidic; colourless solid, the anhydride of $\mathrm{HNO_3}$

Only the two lowest oxides are neutral; only N₂O₅ bridges its nitrogens through oxygen.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2022 · 26 June 2022 · Q127Moderate

Example 1 · The p-Block Elements · Nitrogen and Its Compounds

The oxide which contains an odd electron at the nitrogen atom is

N₂O₄ has no bridging oxygen

When two

\mathrm{NO_2}

molecules combine, the unpaired electrons on nitrogen form an N–N bond:

\mathrm{O_2N{-}NO_2}

. The number of bridging oxygen atoms is zero.

N₂O and NO are neutral, not acidic

Of the oxides of nitrogen, only

\mathrm{N_2O}

and

\mathrm{NO}

are neutral; they form no acid with water.

\mathrm{N_2O_3}

\mathrm{NO_2}

\mathrm{N_2O_4}

and

\mathrm{N_2O_5}

are acidic.

Drill 8 more on oxides of nitrogen: oxidation states, structures and nature

Concept 2 of 2: Preparing dinitrogen and nitric acid, and the nitrogen tests

Dinitrogen is made by bringing nitrogen in −3 and +3 together: ammonium nitrite breaks up into N₂ and water. Very pure N₂ comes from heating an azide. Once made, N₂ is very unreactive because the N≡N bond needs 946 kJ/mol to break; even with oxygen it reacts only at very high temperatures, since forming NO is endothermic. Nitric acid is made the other way round, by oxidising ammonia step by step to NO, then NO₂, which water turns into HNO₃. The brown ring test catches the same NO: it bonds to iron(II) and gives a brown complex.

Definition

Dinitrogen: $\mathrm{NH_4Cl + NaNO_2 \rightarrow N_2 + 2H_2O + NaCl}$ ; $\mathrm{(NH_4)_2Cr_2O_7 \xrightarrow{\Delta} N_2 + 4H_2O + Cr_2O_3}$ ; very pure from $\mathrm{Ba(N_3)_2 \rightarrow Ba + 3N_2}$ .
$\mathrm{N_2 + O_2 \rightleftharpoons 2NO}$ is endothermic and needs about 2000 K or lightning, so the gases of air do not react normally.
Ostwald process: $\mathrm{4NH_3 + 5O_2 \xrightarrow{Pt/Rh} 4NO + 6H_2O}$ ; $\mathrm{2NO + O_2 \rightleftharpoons 2NO_2}$ ; $\mathrm{3NO_2 + H_2O \rightarrow 2HNO_3 + NO}$ .
Brown ring test: $\mathrm{NO_3^- + 3Fe^{2+} + 4H^+ \rightarrow NO + 3Fe^{3+} + 2H_2O}$ , then $\mathrm{[Fe(H_2O)_6]^{2+} + NO \rightarrow [Fe(H_2O)_5(NO)]^{2+} + H_2O}$ , the brown ring.
Nessler's reagent ( $\mathrm{K_2[HgI_4]}$ in alkali) gives a brown precipitate with ammonia.
Gases from common reactions: $\mathrm{KMnO_4 + HCl}$ gives $\mathrm{Cl_2}$ ; $\mathrm{Al + NaOH + H_2O}$ gives $\mathrm{H_2}$ ; heating $\mathrm{NaNO_3}$ gives $\mathrm{NaNO_2 + O_2}$ .
Industrial processes: Haber for $\mathrm{NH_3}$ , Ostwald for $\mathrm{HNO_3}$ , Contact for $\mathrm{H_2SO_4}$ , Hall–Héroult for aluminium.

Dinitrogen, the Ostwald process and the brown ring

\mathrm{NH_4Cl + NaNO_2 \rightarrow N_2 + 2H_2O + NaCl} \qquad \mathrm{3NO_2 + H_2O \rightarrow 2HNO_3 + NO} \qquad \mathrm{[Fe(H_2O)_6]^{2+} + NO \rightarrow [Fe(H_2O)_5(NO)]^{2+} + H_2O}

Worked example

Write the three steps of the Ostwald process for nitric acid, starting from ammonia. Which gas is formed again in the last step and sent back?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 6 Apr 2026 Shift 2 · Q45Moderate

Example 2 · The p-Block Elements · Nitrogen and Its Compounds

Treatment of a gas ' X ' with a freshly prepared ferrous sulphate solution gives a compound ' Y ' as a brown ring. The compounds X and Y are.

Air does not form NO because the reaction is endothermic

Nitrogen and oxygen need about 2000 K, as in lightning, to form NO, because

\mathrm{N_2 + O_2 \rightarrow 2NO}

absorbs heat. The reason is not that nitrogen oxides are unstable.

The brown ring holds NO, not NO₂

Iron(II) reduces nitrate to nitric oxide, and NO bonds to iron as

\mathrm{[Fe(H_2O)_5(NO)]^{2+}}

. No complex of

\mathrm{NO_2}

\mathrm{N_2O}

forms.

Dilute nitric acid on lead sulphide gives NO, not N₂O

\mathrm{3PbS + 8HNO_3 \rightarrow 3Pb(NO_3)_2 + 2NO + 3S + 4H_2O}

. The products are lead nitrate, sulphur, nitric oxide and water; nitrous oxide is not formed.

Drill 8 more on preparing dinitrogen and nitric acid, and the nitrogen tests

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Preparing dinitrogen and nitric acid, and the nitrogen tests
Dinitrogen, the Ostwald process and the brown ring
$\mathrm{NH_4Cl + NaNO_2 \rightarrow N_2 + 2H_2O + NaCl} \qquad \mathrm{3NO_2 + H_2O \rightarrow 2HNO_3 + NO} \qquad \mathrm{[Fe(H_2O)_6]^{2+} + NO \rightarrow [Fe(H_2O)_5(NO)]^{2+} + H_2O}$

Reference tables (1)

Oxides of nitrogen: oxidation states, structures and nature6 rows

Oxide	Oxidation state of N	Structure	Nature
$\mathrm{N_2O}$	+1	Linear N≡N–O; one N–N bond	Neutral; colourless gas
$\mathrm{NO}$	+2	N=O with one unpaired electron	Neutral; colourless gas
$\mathrm{N_2O_3}$	+3	O=N–NO₂; one N–N bond	Acidic; blue solid
$\mathrm{NO_2}$	+4	Bent, odd electron on N; one N=O and one N–O	Acidic; brown gas The odd-electron oxide that dimerises to N₂O₄.
$\mathrm{N_2O_4}$	+4	O₂N–NO₂; one N–N bond, no bridging O	Acidic; colourless
$\mathrm{N_2O_5}$	+5	O₂N–O–NO₂; one N–O–N bridge, no N–N bond	Acidic; colourless solid, the anhydride of $\mathrm{HNO_3}$

Only the two lowest oxides are neutral; only N₂O₅ bridges its nitrogens through oxygen.

Watch out for (5)

N₂O₄ has no bridging oxygen
→ Oxides of nitrogen: oxidation states, structures and nature
N₂O and NO are neutral, not acidic
→ Oxides of nitrogen: oxidation states, structures and nature
Air does not form NO because the reaction is endothermic
→ Preparing dinitrogen and nitric acid, and the nitrogen tests
The brown ring holds NO, not NO₂
→ Preparing dinitrogen and nitric acid, and the nitrogen tests
Dilute nitric acid on lead sulphide gives NO, not N₂O
→ Preparing dinitrogen and nitric acid, and the nitrogen tests

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