Occurrence

About 78% of the earth's atmosphere by volume is dinitrogen (N2) gas, making nitrogen the most abundant element of Group 15 by far. Combined nitrogen is found in the earth's crust as sodium nitrate (Chile saltpetre, NaNO3) and potassium nitrate (Indian saltpetre, KNO3), and it is also an essential constituent of proteins and nucleic acids in living systems. Phosphorus, the 11th most abundant element in the earth's crust, never occurs free; it is found combined as phosphate minerals — fluorapatite [Ca5(PO4)3F], chlorapatite [Ca5(PO4)3Cl] and hydroxyapatite [Ca5(PO4)3OH] — which are also the principal source of phosphorus for fertiliser manufacture. The heavier members arsenic, antimony and bismuth are comparatively scarce elements and occur mainly as sulphide ores (for example, arsenic a …