Chemistry · Ch 14 — The p-Block Elements

Oxides of Nitrogen

14.4

Oxides of Nitrogen

Nitrogen is unusual in forming a whole family of oxides spanning almost every oxidation state from $+1$ to $+5$ . Table 7.3 lists these oxides together with their formulas, oxidation states, typical laboratory preparations, and their physical appearance and acid–base character; Table 7.4 then sets out their Lewis-dot/resonance structures and measured bond parameters.

Name	Formula	Oxidation state of N	Common method of preparation	Appearance / nature
Dinitrogen oxide [Nitrogen(I) oxide]	$N_2O$	$+1$	$NH_4NO_3 \xrightarrow{\text{Heat}} N_2O + 2H_2O$	colourless gas, neutral
Nitrogen monoxide [Nitrogen(II) oxide]	$NO$	$+2$	$2NaNO_2 + 2FeSO_4 + 3H_2SO_4 \rightarrow Fe_2(SO_4)_3 + 2NaHSO_4 + 2H_2O + 2NO$	colourless gas, neutral
Dinitrogen trioxide [Nitrogen(III) oxide]	$N_2O_3$	$+3$	$2NO + N_2O_4 \xrightarrow{250\ K} 2N_2O_3$	blue solid, acidic
Nitrogen dioxide [Nitrogen(IV) oxide]	$NO_2$	$+4$	$2Pb(NO_3)_2 \xrightarrow{673\ K} 4NO_2 + 2PbO + O_2$	brown gas, acidic
Dinitrogen tetroxide [Nitrogen(IV) oxide]	$N_2O_4$	$+4$	$2NO_2 \underset{\text{Heat}}{\overset{\text{Cool}}{\rightleftharpoons}} N_2O_4$	colourless solid/liquid, acidic
Dinitrogen pentoxide [Nitrogen(V) oxide]	$N_2O_5$	$+5$	$4HNO_3 + P_4O_{10} \rightarrow 4HPO_3 + 2N_2O_5$	colourless solid, acidic

Reading down this table, the oxidation state of nitrogen rises steadily from $+1$ in $N_2O$ to $+5$ in $N_2O_5$ , and the character of the oxide shifts in step with it: the very lowest oxide, $N_2O$ , is neutral, while every oxide from $N_2O_3$ onward is acidic — consistent with the general rule that a nitrogen oxide becomes more acidic as the oxidation state of nitrogen climbs.

Formula	Structure (resonance)	Bond parameters
$N_2O$	Linear; $N=N=O \leftrightarrow :N\equiv N-O:$ , with lone pairs on the terminal $N$ and terminal $O$	$N$ – $N$ : 113 pm; $N$ – $O$ : 119 pm
$NO$	An odd-electron molecule; $:N=O: \leftrightarrow :N=O:$ , the unpaired electron shifting between the two resonance forms	$N$ – $O$ : 115 pm
$N_2O_3$	Planar; a central $N$ – $N$ single bond (186 pm) links two nitrogens, one bearing a double-bonded $O$ (114 pm, 105 $^\circ$ ) and a lone-pair-bearing $O$ , the other bearing a double-bonded $O$ (121 pm, 130 $^\circ$ ) and lone pairs; a second resonance form redistributes the double bonds among the equivalent oxygens	$N$ – $N$ : 186 pm; $N$ = $O$ : 114 pm (105 $^\circ$ ) and 121 pm (130 $^\circ$ ); overall angle 117 $^\circ$
$NO_2$	Angular, with the odd electron centred on nitrogen; one $N=O$ double bond and one $N$ – $O$ single bond, interchanged in the second resonance form	$N$ – $O$ : 120 pm; $O$ – $N$ – $O$ : 134 $^\circ$
$N_2O_4$	Planar; two nitrogens joined by a central $N$ – $N$ single bond (175 pm), each carrying one double- and one single-bonded oxygen, with the double bonds redistributed in the second resonance form	$N$ – $N$ : 175 pm; $N$ – $O$ : 121 pm (135 $^\circ$ )

Table 7.3Oxides of Nitrogen

Name	Formula	Oxidation state of nitrogen	Common methods of preparation	Physical appearance and chemical nature
Dinitrogen oxide [Nitrogen(I) oxide]	N2O	+1	NH4NO3 --Heat--> N2O + 2H2O	colourless gas, neutral
Nitrogen monoxide [Nitrogen(II) oxide]	NO	+2	2NaNO2 + 2FeSO4 + 3H2SO4 → Fe2(SO4)3 + 2NaHSO4 + 2H2O + 2NO	colourless gas, neutral
Dinitrogen trioxide [Nitrogen(III) oxide]	N2O3	+3	2NO + N2O4 --250K--> 2N2O3	blue solid, acidic
Nitrogen dioxide [Nitrogen(IV) oxide]	NO2	+4	2Pb(NO3)2 --673K--> 4NO2 + 2PbO + O2	brown gas, acidic

Table 7.4Structures of Oxides of Nitrogen (Lewis dot / resonance structures and bond parameters). NOTE: the source prints these as hand-drawn Lewis-dot resonance diagrams (lone-pair dots, double/triple-bond lines, resonance double-arrows) rather than plain text; the 'Resonance Structures' cells below are a best-effort descriptive transcription, not a character-exact rendering — see chunk notes.

Formula	Resonance Structures	Bond Parameters
N2O	N=N=O (lone pairs on terminal N and terminal O) <-> :N≡N–O: (N≡N triple bond, N–O single bond, lone pairs on terminal N and O)	N–N: 113 pm; N–O: 119 pm; Linear
NO	:N=O: <-> :N=O: (odd-electron molecule; the unpaired electron/dot is shown shifting between N and O in the two resonance forms)	N–O: 115 pm
N2O3	Planar structure: a central N–N single bond (186 pm) joins two nitrogen atoms; one N carries a terminal O via double bond and a second O with lone pairs (O–N angle 105°, N=O 114 pm); the other N carries two O atoms, one via double bond (121 pm) at 130° and lone pairs shown on all O atoms; a second resonance form <-> shows the double bonds/lone pairs redistributed among the equivalent O atoms	O–N angle 105° (114 pm); N–N 186 pm; O–N angle 130° (121 pm); angle 117°; Planar
NO2	Angular molecule: central N (bearing the odd/unpaired electron) bonded to two O atoms — one via N=O double bond, one via N–O single bond with lone pairs; second resonance form <-> shows the double bond on the other O atom	N–O: 120 pm; O–N–O angle: 134°; Angular
N2O4	Planar structure: two N atoms joined by a central N–N single bond (175 pm), each N also bonded to two O atoms (one double, one single bond per N, lone pairs shown on all O atoms); second resonance form <-> redistributes the double bonds among the four O atoms	N–N: 175 pm; N–O: 121 pm at 135° on one side, N–O 121 pm on other side; Planar

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