347In this sense, red phosphorus is not an allotrope, but rather an intermediate phase
between the white and violet phosphorus, and most of its properties have a range of
values. For example, freshly prepared, bright red phosphorus is highly reactive and ignites
at about 300 °C, though it is still more stable than white phosphorus, which ignites at about
30 °C. After prolonged heating or storage, the color darkens; the resulting product is more
stable and does not spontaneously ignite in air.
Violet Phosphorus
Violet phosphorus is a form of phosphorus that can be produced by day-long annealing of
red phosphorus above 550 °C. In 1865, Hittorf discovered that when phosphorus was
recrystallized from molten lead, a red/purple form is obtained. Therefore this form is
sometimes known as "Hittorf's phosphorus" (or violet or α-metallic phosphorus).
Black Phosphorus
Black phosphorus is the least reactive allotrope and the thermodynamically stable form
below 550 °C. It is also known as β-metallic phosphorus and has a structure somewhat
resembling that of graphite. High pressures are usually required to produce black
phosphorus, but it can also be produced at ambient conditions using metal salts as
catalysts.
Properties of some allotropes of phosphorus
Form white(α) white(β) violet black
Symmetry Body-centered cubic Triclinic Monoclinic Orthorhombic
Pearson symbol^ aP24 mP84 oS8
Space group I43m P1 No.2 P2/c No.13 Cmca No.64
Density (g/cm^3 ) 1.828 1.88 2.36 2.69
Bandgap (eV) 2.1^ 1.5 0.34
Refractive index 1.8244^ 2.6 2.4Isotopes
Twenty-three isotopes of phosphorus are known, including all possibilities from^24 P up to
(^46) P. Only (^31) P is stable and is therefore present at 100% abundance. The half-integer
nuclear spin and high abundance of^31 P make phosphorus-31 NMR spectroscopy a very
useful analytical tool in studies of phosphorus-containing samples.
Two radioactive isotopes of phosphorus have half-lives that make them useful for scientific
experiments.^32 P has a half-life of 14.262 days and^33 P has a half-life of 25.34 days.
Biomolecules can be "tagged" with a radioisotope to allow for the study of very dilute
samples.
Radioactive isotopes of phosphorus include
32 P, a beta-emitter (1.71 MeV) with a half-life of 14.3 days, which is used routinely
in life-science laboratories, primarily to produce radiolabeled DNA and RNA
probes, e.g. for use in Northern blots or Southern blots. Because the high energy
beta particles produced penetrate skin and corneas, and because any^32 P
ingested, inhaled, or absorbed is readily incorporated into bone and nucleic acids,
Occupational Safety and Health Administration in the United States, and similar
institutions in other developed countries require that a lab coat, disposable gloves
and safety glasses or goggles be worn when working with^32 P, and that working
directly over an open container be avoided in order to protect the eyes. Monitoring