code: public int numberOfUnplacedHeavyAtoms(IAtomContainer ac) {
          int nUnplacedHeavyAtoms = 0;
          for (int i = 0; i < ac.getAtomCount(); i++) {
              if (isUnplacedHeavyAtom(ac.getAtom(i))) {
                  nUnplacedHeavyAtoms++;
              }
          }
          return nUnplacedHeavyAtoms;
      }
docs: Gets numberOfUnplacedHeavyAtoms (no Flag ISPLACED, no Hydrogens)
  
  @param  ac AtomContainer
  @return #UnplacedAtoms