where n = 1, 2,...9. Lesser amounts of phosphorus
1928). Phosphorus was subsequently named
trioxide (P4O6) and phosphorus tetraoxide
from the Greek word for "light bearer" since it
glowed in the dark.
(P4O8) are products, depending on the availabili-
ty of oxygen. Phosphorus trioxide is responsible
for the garlic-like smell immediately noticeable
Properties of white phosphorus
Phosphorus occurs in both organic and inor-
when P4 is exposed to air. Phosphorus pentoxide
ganic forms in nature almost exclusively in oxi-
is one of the most hygroscopic substances
dation state +5. Mean phosphorus concentra-
known. It will extract moisture from the atmo-
tions in soil and crustal rocks are 430 and 1000
sphere; the dense white cloud formed following
mg/kg, respectively (Sposito 1989), making it
the bursting of a P4 projectile is actually moisture
the twelfth most abundant element in the litho-
condensing on oxidized white phosphorus.
sphere. Phosphorus plays vital roles in biological
Phosphorus oxides hydrolyze in water to form
systems in compounds such as ATP (adenosine
various oxyacids.
triphosphate), phospholipids, DNA (deoxyribo-
The reaction between P4 vapor and oxygen
nucleic acid) and phosphate buffers, and it is
has several anomalies that have intrigued scien-
found in the bones and teeth of vertebrates as
tists for over three hundred years (Semenov
calcium phosphate.
1959). The reaction is characterized by a greenish
Elemental phosphorus (oxidation state 0) does
glow that occurs between two sharply defined
not exist in nature; it is prepared commercially
limits of oxygen pressure, which diverge as P4
pressure increases. Outside the limits, the rate is
(SiO2) and coke in an electric furnace. White
so slow that it is undetectable. The lower limit is
phosphorus is a wax-like solid that ignites spon-
affected by the size and surface condition of the
taneously in air between 30 and 40C. Its molecu-
reaction vessel and the presence of other gases.
lar structure is that of a regular tetrahedron with
a phosphorus atom at each apex and a PP dis-
Table 1. Chemical and physical properties of white
tance of 0.22 nm. Since its molecular formula is
phosphorus.
P4, its molecular weight is 124 amu. The high re-
activity of white phosphorus is attributed to the
Property
Value
Source*
extreme bond strain involved in the bonding of
Molecular structure
P4
a
four phosphorus atoms in a tetrahedral configu-
Molecular weight
124 amu
a
ration (Van Wazer 1958). Some chemical and
Physical state at STP
Solid
a
physical properties of white phosphorus are giv-
44C
Melting point
a
en in Table 1.
280C
Boiling point
a
Elemental phosphorus exists in several allo-
Appearance
Waxy
a
Transparent
tropic forms: white, red and black. White phos-
Colorless (pure)
phorus is sometimes referred to as yellow phos-
Mohs hardness
0.5
a
phorus since the commercially available grade
Density (20C)
1.8 g/cm3
a
has a slight yellow color due to trace amounts of
Vapor pressure (25C)
0.043 torr
a
Viscosity (50C)
red phosphorus as an impurity. Pure white phos-
1.69 cP
b
Solubility
phorus is colorless. The crystal structure is cubic;
in water (15C)
2.4 mg/L
c
a unit cell (a = 1.85 nm) contains 56 P4 tetrameric
3 mg/L
d
molecules. The structure of red phosphorus is
in carbon disulfide (20C)
13 106 mg/L
a
variable. Several polymeric forms exist. Red
in diethyl ether (20C)
7,000 mg/L
a
in benzene (20C)
phosphorus, which is less reactive than white
26,000 mg/L
a
in mineral oil
12,000 mg/L
d
phosphorus, is also used by the Army as a smoke
munition. Red phosphorus is nontoxic. Black
Octanolwater partition coefficient
1200
c
phosphorus is similar in structure to graphite. It
2040C
Autoignition temperature
e
is the most stable form of elemental phosphorus;
Latent heat of fusion (44C)
20 J/g
a
Latent heat of sublimation (25C)
it does not spontaneously ignite under normal
455.2 J/g
a
Latent heat of vaporization (287C)
545 J/g
a
temperatures and humidities.
Heat of combustion
24,000 J/g
a
The oxidation of P4 has been studied exten-
675C, 80 atm
Critical point
a
sively (Mellor 1928, Van Wazer 1958). The final
* aMellor (1928)
dStich (1953)
product of P4 oxidation is phosphorus pentoxide
bKirk Othmer (1982)
eDainton and Bevington (1946)
(P4O10) via lower oxidation state oxides P4On,
cSpanggord et al. (1985) fVan Wazer (1958)
2
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