Comparisons of Pnictide Elements

The existence of the lighter pnictide elements was recognised at much later dates than the heavier ones  

Nitrogen is unique among the pnictide elements because it exists solely as N2 molecules, in the gaseous, liquid and solid states. In all these forms, the element is colourless, odourless, non-inflammable and non-toxic. Nitrogen, N=N, forms the most inert diatomic molecule known. 

The remaining pnictide elements, because of their comparative reluctance to form multiple bonds, all prefer to catenate and consequently exist in forms more highly polymerised than diatomic nitrogen. Elevated temperatures are required to break these polymeric elements into diatomic species. 

Arsenic and antimony exist in yellow crystalline forms which contain tetrahedral AS4 or Sb4 units similar to the P4 units of white phosphorus. These tetrahedral molecules, which pack together into cubic lattices in the solid state, persist in CS2 solution and if the elements are vaporised. 

On heating to higher temperatures, the tetraatomic vapours dissociate to diatomic species and eventually to single atoms. This dissociation takes place more readily with increasing pnictide atomic weight (arsenic is completely dissociated to As at 1700°C). The tetraatomic forms or the vapours from any of the allotropes of P, As or Sb are all extremely toxic either by inhalation or by ingestion. The red, black or metallic forms, unless vaporised, are, on the other hand, much less toxic. 

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