Atomic Mass of Nitrogen

Nitrogen's 14.007 g/mol is nearly all ¹⁴N, with just 0.36 % ¹⁵N, so the standard atomic weight sits very close to a whole number. It is the number that converts between a compound and its nitrogen content: urea, CH₄N₂O at 60.06 g/mol, carries 2 × 14.007 = 28.014 g of nitrogen per mole, which is 46.6 % N — the figure printed as the first number of a fertiliser's N-P-K grade.

The same division runs through environmental reporting, where ammonia, nitrate and nitrite are all normalised "as N" so that they can be added into a single total nitrogen figure. Nitrate at 62.00 g/mol is 22.6 % nitrogen, so the drinking-water limit of 10 mg/L as N is 44.3 mg/L as NO₃⁻ — the same water, two numbers, and a standard source of confusion between American and European reporting conventions.