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White phosphorus was originally made commercially for the match industry in the 19 th century, by distilling off phosphorus vapor from precipitated phosphates, mixed with ground coal or charcoal, [link] . The precipitated phosphates were made from ground-up bones that had been de-greased and treated with strong acids. This process is, however, obsolete due to the submerged-arc furnace for phosphorus production was introduced to reduce phosphate rock. Calcium phosphate (phosphate rock) is heated to 1200 - 1500 °C with SiO 2 and coke (impure carbon) to produce vaporized tetraphosphorus, P 4 .

Physical properties

The physical properties of the Group 15 elements ( [link] ) encompasses a gas (N 2 ), a non-metallic solid (P 4 ), metalloids (As and Sb), and a metal (Bi).

Selected physical properties of the Group 15 elements.
Element Mp (°C) Bp (°C) Density (g/cm 3 )
N -210.00 -195.79 1.251 g/L (0 °C @ 101.325 kPa)
P 44.2 (white), 610 (black) 280.5 (white), 416 - 590 (sub., red), 620 (sub, violet) 1.823 (white), 2.2 - 2.34 (red), 2.36 (violet), 2.69 (black)
As 817 615 (sub.) 5.727
Sb 630.63 1587 6.697 (solid), 6.53 (liquid)
Bi 271.5 1564 9.78 (solid), 10.05 (liquid)

Vapor phase

Nitrogen forms a dimer in the vapor phase with a triple bond ( [link] ). In the vapor phase above 800 °C tetraphosphorus (P 4 ) is partially dissociated to P 2 .

Molecular orbital diagram for the formation of N 2 .

Solid state

Phosphorus forms a number of allotropes with very different properties ( [link] ). Red phosphorus is an intermediate phase between the white and violet forms. Scarlet phosphorus is obtained by allowing a solution of white phosphorus in carbon disulfide to evaporate in sunlight. Black phosphorus is formed by heating white phosphorus under high pressures ( ca . 12,000 atmospheres).

Images of (left to right) white, red, violet and black allotropes of phosphorus.

White phosphorus has two forms, low-temperature β form and high-temperature α form; both of which contain the P4 tetrahedron ( [link] ). White phosphorus is the least stable, the most reactive, most volatile, less dense, and most toxic of the allotropes.

The structure of the P 4 molecule in white phosphorus.

The structural relationship between white and red phosphorus involves breaking one of the P-P bonds in the P 4 unit and forming a bond with a neighboring tetrahedron to give a chain structure ( [link] ). Red phosphorus is formed by heating white phosphorus to 250 °C or by exposing white phosphorus to sunlight. Actually red phosphorus is not a single allotrope, but rather an intermediate phase between the white and violet phosphorus, and most of its properties have a range of values ( [link] ).

Crystal unit cell of red phosphorus.

Violet phosphorus ( [link] ) is the thermodynamic stable form of phosphorus that is produced by heating red phosphorus above 550 °C. Due to the synthesis being developed by Johann Hittorf ( [link] ) it is sometimes known as Hittorf's phosphorus .

Structure of violet (Hittorf’s) phosphorus.
German physicist Johann Wilhelm Hittorf (1824 - 1914).

Black phosphorus is the least reactive allotrope and the thermodynamic stable form below 550 °C. It is also known as β-metallic phosphorus and has a structure somewhat resembling that of graphite ( [link] ).

Crystal structure of black phosphorus.

In a similar manner to phosphorus, arsenic has several allotropes some of which a structurally related to those of phosphorus. Grey arsenic has a structure similar to black phosphorus ( [link] ). Yellow arsenic (As 4 ) is soft and waxy with a structure similar to too P 4 ( [link] ). Finally, black arsenic is similar in structure to red phosphorus ( [link] ). Antimony and bismuth are both traditional metals and have trigonal hexagonal structures ( a = 4.299, c = 11.25 Å, and a = 4.537, c = 11.838 Å, respectively).

Bibliography

  • V. Biringuccio, The Pirotechnia of Vannoccio Biringuccio: The Classic Sixteenth-Century Treatise on Metals and Metallurgy , Dover Publications (1990).

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Source:  OpenStax, Chemistry of the main group elements. OpenStax CNX. Aug 20, 2010 Download for free at http://cnx.org/content/col11124/1.25
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