Phosphorus atomic weight

Phosphorus has only one stable isotope, J P, and accordingly (p. 17) its atomic weight is known with extreme accuracy, 30.973 762(4). Sixteen radioactive isotopes are known, of which P is by far the most important il is made on the multikilogram scale by the neutron irradiation of S(n,p) or P(n,y) in a nuclear reactor, and is a pure -emitter of half life 14.26 days, 1.7()9MeV, rntan 0.69MeV. It finds extensive use in tracer and mechanistic studies. The stable isotope has a nuclear spin quantum number of and this is much used in nmr spectroscopy. Chemical shifts and coupling constants can both be used diagnostically to determine structural information. 

The monomer 19 can also be polymerized using analogous methods of initiation to those employed in organic polymer science. Radical initiators afford regioirregular polymers, whereas anionic initiators add selectively to the phosphorus atom of the P=C bond and thus yield a regioregular polymer [85]. The product of the initial addition of MeLi across the P=C bond, Mes(Me)P-CPh2Li, was identified spectroscopically. The polymers obtained from anionic initiation are spectroscopically identical to those obtained from the thermolysis. Reasonable molecular weights (ca. 5000-10,000 g mol 0 are obtained when methyllithium is used as an initiator. 

Figure 1.9 Results of a chromatographic examination of the hydrolysis products of Graham s salt at pH 4 and 90 °C (a) high-molecular-weight polyphosphates (b) cyclic phosphates containing four to six phosphorus atoms (c) cyclotriphosphate (d) pyrophosphate (e) tripolyphosphate (f) linear polyphosphates containing four to 15 phosphorus atoms (g) orthophosphate (Van Wazer, 1958).

Figure 2.2 Separation of PolyPs of low molecular weight by ion-exchange chromatography on Dowex 1 XIO in a KC1 gradient. The numbers 1-12 represent the number of phosphorus atoms in the PolyP molecules constituting the various fractions (Matsuhashi, 1963).

It has already been emphasized that within a family of non transition elements, metallic character increases with increase in atomic number, atomic weight, and atomic size. There is no better illustration of this trend than Periodic Group Vb the lightest members of the group, nitrogen (Z = 7), and phosphorus (Z — 15), are typical nonmetals, whereas the heaviest member, bismuth (Z = 83) is a typical metal. The remaining members, arsenic (Z = 33) and antimony (Z 51) are intermediate in character and are often appropriately called metalloids. 

The atomic heats are therefore 5-83, 5-74 and 6-26 over the three ranges of temperature (1), (2), (3). There is a slight deviation from Dulong and Petit s law 9 at the lower temperatures, in the same sense as that met with in the case of the elements carbon, boron and silicon. But although phosphorus has a relatively low atomic weight, it also has a low melting-point, and the atomic heat as usual assumes the normal value at temperatures near the melting-point. 

Historical.—In his tables published in 1818 Berzelius4 gave 31-36 as the atomic weight of phosphorus. Other values obtained before and after this time did not agree even so well as this with the value accepted to-day. The reactions principally employed in this early period were the displacement of gold and silver from their chloride and sulphate respectively by elementary phosphorus. These reactions,... 

In the more recent determinations which have led to the accepted value of the atomic weight the method of decomposition of the halides has been followed with the aid of all the present knowledge as to the proper conditions for the conversion of halogen hydrides into silver halides. An interesting method has also been worked out by which silver phosphate is converted into the bromide. The results obtained by this method, which will be described first, give additional weight to those obtained from the phosphorus halides. 

The preceding method is open to the criticism that silver orthophosphate contains only 7-4 per cent, of phosphorus. Phosphorus tribromide is somewhat better in this respect, containing 11-5 per cent, of phosphorus but on the other hand the preparation and quantitative decomposition of this compound in a manner suitable for atomic weight determinations present great difficulties, the nature of which is apparent from the following narrative.1... 

WEIGHTS OF PHOSPHORUS TRIBROMIDE CORRESPONDING TO THREE PARTS BY WEIGHT OF SILVER AND OF SILVER BROMIDE, AND VALUES OF THE ATOMIC WEIGHT OF PHOSPHORUS. 

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