Grammar for Atoms:

<atom id=<!--any String object-->"a1" elementType=<!--any String object-->"O" hydrogenCount=<!--any String object-->"1"> </atom>

id is of class AtomID which only contains a String. It does no processing on the String, so any String object is valid.
elementType is of class ElementType and is structured identically to AtomID.
hydrogenCount is of class HydrogenCount, and is also structured identically to AtomID and ElementType.