Six-Element Lab
The six atoms that build 70% of known compounds
Summary
Carbon, hydrogen, oxygen, nitrogen, sulfur and fluorine: with these six atoms you can build water, ammonia, sulfuric acid, the building block of Teflon, caffeine, aspirin and glucose. 72 molecules are waiting for you in the lab. 70% of the 125 million compounds in the PubChem database are made from these six elements alone: no other set of six produces so many molecules.
In depth
These six elements were not chosen at random. When the 124.9 million compounds in PubChem are counted one by one, among every pair of elements that can be added to the C-H-O-N core, the sulfur + fluorine pair comes first with 87.7 million compounds, ahead of sulfur + chlorine (84.8 million) in second place and CHNOPS, the six elements of life (71.5 million). The reason lies in valence: carbon forms 4 bonds to build chains and rings, nitrogen forms 3 to branch, oxygen forms 2 to bridge, and hydrogen and fluorine form 1 to cap the chain. Sulfur is a rare "flexible" atom that can form 2, 4 or 6 bonds. The lab does the bond bookkeeping live for the atoms you drop in: the total bond ends, the bonds needed and the number of rings + π bonds (the degree of unsaturation) tell you whether a composition is possible on paper.
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