code: @Override
      public String getSymbol() {
          if (atomicNumber == null)
              return null;
          if (atomicNumber == 0)
              return "R";
          return Elements.ofNumber(atomicNumber).symbol();
      }
docs: Returns the element symbol of this element.
  
  @return The element symbol of this element. Null if unset.
  
  @see    #setSymbol