code: IAtom getUnplacedHeavyAtom(IAtomContainer molecule) {
          for (IAtom atom : molecule.atoms()) {
              if (isUnplacedHeavyAtom(atom))
                  return atom;
          }
          return null;
      }
docs: Find the first unplaced atom.
  
  @param molecule molecule being built
  @return an unplaced heavy atom, null if none.