code: public IAtom configure(IAtom atom) {
          IIsotope isotope;
          if (atom.getMassNumber() == null)
              return atom;
          else
              isotope = getIsotope(atom.getSymbol(), atom.getMassNumber());
          if (isotope == null)
              throw new IllegalArgumentException("Cannot configure an unrecognized element/mass: " + atom.getMassNumber() + " " + atom);
          return configure(atom, isotope);
      }
docs: Configures an atom. Finds the correct element type
  by looking at the atoms element symbol. If the element symbol is not recognised, it will
  throw an {@link IllegalArgumentException}.
  
  @param  atom  The atom to be configured
  @return       The configured atom