10. Playing with a Full Deck:
       The Periodic Table
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       Group VA; The Nitrogen Cycle

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


Ammonia is a base because the nitrogen atom has a lone electron pair that can bind a proton:


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