Page 212 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

Main Group Elements 21

Phosphorus exists in several allotropic forms, but only synthesized; however, not all have been well characterized
two are considered: the white and the red. The phospho- because of their instability. The shapes of the halides in
rus vapor generated by the reduction of apatite is highly the gas phase are as follows (X = F, Cl, Br, I):
flammable, so it is condensed under water to give white X
X X
phosphorus, a low-density (1.82 g/cm 3 ), low-melting X
◦
(mp 44 C), highly reactive waxy species in which the four P X P P X P
X X X
phosphorus atoms occupy the corners of a tetrahedron. X X X
White phosphorus stored under water for a long time is Trihalides Diphosphorus Pentahalides
(trigonal pyramidal) tetrahalides (trigonal
gradually converted to the less toxic, more stable poly- bipyramidal)
meric red phosphorus.
Pentahalides, however, may exist as ions in the crystalline
P P P P
−
+
storage phase, for example, [PCl ] [PCl ].
P P P P P P P P 4 6
P P P P
White phosphorus Red phosphorus D. Oxides
(known configuration) (proposed configuration)
Only the two most important stable binary oxides, P 4 O 6
A. Phosphides and P 4 O 10 , are considered, but many others are known,
even the diatomic species PO, whose electronic spectrum
A majority of the elements in the periodic table form sta-
has been characterized.
ble binary compounds with phosphorus. If the element is
P 4 O 6 hydrolyzes in cold water to yield H(O)P(OH) 2 ,
a metal, the binary compound formed is referred to as a
phosphorous acid (phosphonic acid), a diprotic acid:
phosphide. Many stoichiometries are known, and some
O
metals form several different phosphides. Nickel, for ex-
ample, forms eight known compounds. P 4 O 6 6H 2 O 4H(O)P(OH) 2 , P O H
Phosphides can be produced by reaction of red phos- H O H
phorus with a metal at an elevated temperature in an inert Phosphonic acid
atmosphere: The most important oxide P 4 O 10 , is often referred to as
phosphorus pentoxide. It hydrolyzes in water to yield
3Ca + 2P → Ca 3 P 2
H 3 PO 4 , orthophosphoric acid, a triprotic acid:
Al + P → AlP
O
Most phosphides are industrially worthless; however,
a few are valued for some special property. Aluminum P 4 O 10 6H 2 O 4H 3 PO 4 , P O H
H O
phosphidefindsapplicationasasemiconductor,andCa 3 P 2 O H
is employed in sea flares to produce flammable phosphine Orthophosphoric acid
gas (PH 3 ):
E. Oxoacids and Oxoanions
Ca 3 P 2 + 6H 2 O → 2PH 3 + 3Ca(OH) 2
4PH 3 (g) + 8O 2 (g) → P 4 O 10 (s) + 6H 2 O (g) Phosphorus forms more oxoacids than any other element,
and the number of oxoanions and oxo salts ranks second
only to silicon. Phosphoric acid, H 3 PO 4 , ranks seventh
B. Phosphines among the top 50 chemicals in production, and sodium
The hydrides of phosphorus are called phosphines. The tripolyphosphate, Na 5 P 3 O 10 , ranks just outside the top 50.
general formula for this homologous series of very poor Na 5 P 3 O 10 may be regarded as a salt of triphosphoric acid,
H 5 P 3 O 10 , whose structure is as follows:
thermal stability is P n H n+2 (n = 1–6). Other, even less
stable homologous series are also known. O O O
The most important phosphine is PH 3 , a colorless, poi-
P P P
sonous, highly reactive gas. PH 3 is used in naval flares, H O O O O H
H O O H O H
and it and organic derivatives can serve as ligands that
bind to metal atoms. Na 5 P 3 O 10 is not formed by the reaction of NaOH with
H 5 P 3 O 10 but, rather, is produced by fusion of the dibasic
C. Halides and monobasic sodium salts of phosphoric acid at a 2:1
ratio:
Three series of phosphorus halides, PX 3 ,P 2 X 4 , and PX 5 ,

are known, and all 12 possible compounds have now been 2Na 2 HPO 4 + NaH 2 PO 4 → Na 5 P 3 O 10 + 2H 2 O