Alternate forms
The order CHNO matches the systematic order of molecular formulas, where elements other than carbon are in alphabetical order.
The order HONC lines each element up with its unpaired valence electrons, which is normally the number of chemical bonds it makes: 1 for hydrogen, 2 for oxygen, 3 for nitrogen, and 4 for carbon. This is less commonly used outside introductory chemistry as HONC is also the molecular formula of isofulminic acid.
The acronym is also sometimes expanded into CHONPS or CHONPS, adding phosphorus and sulfur, the next two most important elements for life.
