Occurrence of The group 15 elements

Figure 1.1a illustrates the relative abundances of the group 15 elements in the Earth’s crust. Naturally occurring N₂ makes up 78% (by volume) of the Earth’s atmosphere (Figure 1.1b) and contains ≈ 0.36% ¹⁵N. The latter is useful for isotopic labelling and can be obtained in concentrated form by chemical exchange processes similar to those exemplified for ¹³C . Because of the availability of N₂ in the atmosphere and its requirement by living organisms (in which N is present as proteins), the fixing of nitrogen in forms in which it may be assimilated by plants is of great importance. Attempts to devise synthetic nitrogen-fixation processes  that mimic the action of bacteria living in root nodules ofleguminous plants have not yet been successful, although N₂ can be fixed by other processes, e.g. its industrial conversion to NH₃ . The only natural source of nitrogen suitably ‘fixed’ for uptake by plants is crude NaNO₃ (Chile saltpetre or sodanitre) which occurs in the deserts of South America.

    Phosphorus is an essential constituent of plant and animal tissue; calcium phosphate occurs in bones and teeth, and phosphate esters of nucleotides are of immense biological significance. Phosphorus occurs naturally in the form of apatites, Ca₅X(PO₄)₃, the important minerals being fluorapatite (X=F), chlorapatite (X = Cl) and hydroxyapatite (X=OH). Major deposits of the apatite-containing ore phosphate rock occur in North Africa, North America, Asia and the Middle East.   Although arsenic occurs in the elemental form, commercial sources of the element are mispickel (arsenopyrite, FeAsS), realgar (As₄S₄) and orpiment (As₂S₃). Native antimony is rare and the only commercial ore is stibnite (Sb₂S₃). Bismuth occurs as the element, and as the ores bismuthinite (Bi₂S₃) and bismite (Bi₂O₃).