The Foundations of Chemistry

NO is a colorless gas that condenses at 152°C to a blue liquid. Gaseous NO is para-
magnetic and contains one unpaired electron per molecule.

Its unpaired electron makes nitric oxide very reactive. Molecules that contain unpaired
electrons are called radicals.NO reacts with O 2 to form NO 2 , a brown, corrosive gas.

2NO(g)
O 2 (g)88n2NO 2 (g)
heat

In recent years, researchers have learned that very low concentrations of NO have impor-
tant roles in a variety of physiological processes, not only in humans, but in such a variety
of animals as barnacles, chickens, trout, and fruit flies. It has recently been found to be
involved in transmission of signals by nerves, blood clotting, local control of blood flow,
the immune system’s ability to kill tumor cells and bacteria, and possibly even memory.

Nitrogen Dioxide and Dinitrogen Tetroxide (
4 Oxidation State)

Nitrogen dioxide is formed by reaction of NO with O 2. It is prepared in the laboratory
by heating heavy metal nitrates.

heat

2Pb(NO 3 ) 2 (s)888n2PbO(s)
4NO 2 (g)
O 2 (g)

The NO 2 molecule is angular. It is represented by resonance structures.

Because each NO 2 molecule contains one unpaired electron, NO 2 readily dimerizes to
form colorless, diamagnetic dinitrogen tetroxide, N 2 O 4 , at low temperatures.

2NO 2 (g) 34 N 2 O 4 (g) H^0 57.2 kJ/mol rxn

brown colorless

The N 2 O 4 molecule is also represented by resonance structures.

N

O

O

N

O

O

N

O

O

N

O

O

N

O

O

N

O

O

N

O

O

N

O

O

N

O

O

nitrogen dioxide

N

O

O

NO NO

24-15 Nitrogen Oxides 961

Nitrogen oxide, or nitric oxide, NO,

mp 163.6°C, bp 151.8°C, bond

distance (1.15 Å) intermediate

between NmO (1.06 Å) and NUO

(1.20 Å).

Nitrogen dioxide, NO 2 , mp

11.20°C, one unpaired electron,

bond length 1.197 Å; brown gas.

Dinitrogen tetroxide, N 2 O 4 , bp 21.2°C, no unpaired electrons,

NXN bond length 1.64 Å, NXO bond length 1.17Å,

OXNXO bond angle 126°.