code: public static MarkedElement markupAtom(IRenderingElement elem, IAtom atom) {
          if (elem == null)
              return null;
          MarkedElement tagElem = markupChemObj(elem, atom);
          tagElem.aggClass("atom");
          return tagElem;
      }
docs: Markup a atom with the class 'atom' and optionally the ids/classes
  from it's properties.
  
  @param elem rendering element
  @param atom atom
  @return the marked element