code: function makeNumIsoH(code, bonds) {
          var valence;
  
          if (code === 15) {
              valence = 0;
          } else if (code === 0) {
              valence = -1;
          } else {
              // num_iso_H should never be negative when valence is set
              valence = Math.max(0, code - bonds);
          }
  
          return [valence, 0, 0, 0];
      }
docs: TODO(SOM): set num_iso_H using valence num_iso_H = [n, 0, 0, 0] where n == -1 if valence is not specified (this is default) n = valence - #bonds if valence is specified