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Black phosphorus is the thermodynamically stable form of phosphorus at room temperature and pressure, with a heat of formation of −39.3 kJ/mol (relative to white phosphorus which is defined as the standard state). [1] It was first synthesized by heating white phosphorus under high pressures (12,000 atmospheres) in 1914.
White phosphorus is the least stable, the most reactive, the most volatile, the least dense and the most toxic of the allotropes. White phosphorus gradually changes to red phosphorus, accelerated by light and heat. Samples of white phosphorus almost always contain some red phosphorus and accordingly appear yellow.
Although phosphorus (15 P) has 22 isotopes from 26 P to 47 P, only 31 P is stable; as such, phosphorus is considered a monoisotopic element. The longest-lived radioactive isotopes are 33 P with a half-life of 25.34 days and 32 P with a half-life of 14.268 days. [3] [4] All others have half-lives of under 2.5 minutes, most under a second.
Of the chemical elements, only 1 element has 10 such stable isotopes, 5 have 7 stable isotopes, 7 have 6 stable isotopes, 11 have 5 stable isotopes, 9 have 4 stable isotopes, 5 have 3 stable isotopes, 16 have 2 stable isotopes, and 26 have 1 stable isotope. [1]
Under standard conditions, red phosphorus is more stable than white phosphorus, but less stable than the thermodynamically stable black phosphorus. The standard enthalpy of formation of red phosphorus is −17.6 kJ/mol. [3] Red phosphorus is kinetically most stable. Being polymeric, red phosphorus is insoluble in solvents. It shows ...
For elements that have multiple allotropes, the reference state usually is chosen to be the form in which the element is most stable under 1 bar of pressure. One exception is phosphorus, for which the most stable form at 1 bar is black phosphorus, but white phosphorus is chosen as the standard reference state for zero enthalpy of formation. [2]
White phosphorus is also the most common, industrially important, and easily reproducible allotrope, and for these reasons is regarded as the standard state of phosphorus. The most stable form is the black allotrope, which is a metallic looking, brittle and relatively non-reactive semiconductor (unlike the white allotrope, which has a white or ...
Although both are called "white phosphorus", in fact two different crystal allotropes are known, interchanging reversibly at 195.2 K. [5] The element's standard state is the body-centered cubic α form, which is actually metastable under standard conditions. [4] The β form is believed to have a hexagonal crystal structure. [5]