An element (or an elementary substance) is matter, the atoms of which are alike in having the same positive charge on the nucleus (or atomic number).
Of course it is not helpful that ‘element’ is used for either ‘elementary substance’ or ‘atom’. The ChEBI solution was to get away from ‘element’. Instead, there are either atoms or elemental molecular entities. These belong to different branches of ontology, so they should not really be confused. Or at least, that was the idea.
In ChEBI, ‘elemental’ applies to any class of molecular entities which consist of only one type of atom, be they mono- or polyatomic. For instance, elemental oxygen can be mono-, di- or triatomic. On the other hand, the oxygen atom can be part of a non-elemental molecular entity.
If there is a scope for confusion, people will get confused. Here’s a question I heard on more than one occasion: what is the difference between monoatomic oxygen and oxygen atom? After all, any form of monoatomic oxygen, viz. oxide(•1−), oxide(2−), or neutral monooxygen, can also be referred to as an ‘oxygen atom’. Ditto any of the monooxygen groups: oxido (—O−), oxo (=O) and oxy (‒O‒). The thing is, they belong to different universes (which, properly, should be made disjoint):
- monoatomic oxygen is a monoatomic entity is a molecular entity
- monooxygen group is a group
- oxygen atom is a atom
- Leigh, G.J., Favre, H.A. and Metanomski, W.V. Principles of Chemical Nomenclature: A Guide to IUPAC Recommendations, Blackwell Science, 1998, p. 3.