 However, people have known for years that the real form of the molecule is P4O10, not P2O5, yet it is not normally called "tetraphosphorus decaoxide".In writing formulas, "ammonia" is NH3 even though nitrogen is more electronegative (in line with the convention used by IUPAC as detailed in Table VI of the red book).Likewise, "methane" is written as CH4 even though carbon is more electronegative (Hill system).