code: public static IDifference difference(IChemObject first, IChemObject second) {
          if (!(first instanceof IAtomType && second instanceof IAtomType)) {
              return null;
          }
          IAtomType firstElem = (IAtomType) first;
          IAtomType secondElem = (IAtomType) second;
          ChemObjectDifference totalDiff = new ChemObjectDifference("AtomTypeDiff");
          totalDiff.addChild(StringDifference.construct("N", firstElem.getAtomTypeName(), secondElem.getAtomTypeName()));
          totalDiff.addChild(BondOrderDifference.construct("MBO", firstElem.getMaxBondOrder(),
                  secondElem.getMaxBondOrder()));
          totalDiff
                  .addChild(DoubleDifference.construct("BOS", firstElem.getBondOrderSum(), secondElem.getBondOrderSum()));
          totalDiff
                  .addChild(IntegerDifference.construct("FC", firstElem.getFormalCharge(), secondElem.getFormalCharge()));
          totalDiff.addChild(AtomTypeHybridizationDifference.construct("H", firstElem.getHybridization(),
                  secondElem.getHybridization()));
          totalDiff.addChild(IntegerDifference.construct("NC", firstElem.getFormalNeighbourCount(),
                  secondElem.getFormalNeighbourCount()));
          totalDiff.addChild(DoubleDifference.construct("CR", firstElem.getCovalentRadius(),
                  secondElem.getCovalentRadius()));
          totalDiff.addChild(IntegerDifference.construct("V", firstElem.getValency(), secondElem.getValency()));
          totalDiff.addChild(IsotopeDiff.difference(first, second));
          if (totalDiff.childCount() > 0) {
              return totalDiff;
          } else {
              return null;
          }
      }
docs: Compare two {@link IChemObject} classes and return the difference as an {@link IDifference}.
  
  @param first  the first of the two classes to compare
  @param second the second of the two classes to compare
  @return an {@link IDifference} representation of the difference between the first and second {@link IChemObject}.