We
have discussed elemental nitrogen, ammonia, and nitric acid, and
phosphorus and the phosphates, in previous chapters. Eighty percent
of our atmosphere is
gas. Although this amount represents only a tiny fraction of the
planet as a whole. it is important because it is concentrated at
the surface, where life evolved.
Nitrogen and phosphorus are essential to living organisms; arsenic,
antimony, and bismuth are not. Nitrogen can form covalently bonded
molecular compounds with carbon and is essential in many biological
molecules. Proteins are longchain polymers of amino acids with the
backbone structure
-N-C-C-N-C-C-N-C-C-N-C-C-
Ammonia is a base because the nitrogen atom has a lone electron
pair that can bind a proton: