 Natural ytterbium is composed of seven stable isotopes : 168Yb , 170Yb , 171Yb , 172Yb , 173Yb , 174Yb , and 176Yb , with 174Yb being the most abundant isotope , at 31 @.@ 8 % of the natural abundance ) . 27 radioisotopes have been observed , with the most stable ones being 169Yb with a half @-@ life of 32 @.@ 0 days , 175Yb with a half @-@ life of 4 @.@ 18 days , and 166Yb with a half @-@ life of 56 @.@ 7 hours . All of its remaining radioactive isotopes have half @-@ lives that are less than two hours and most of these have half @-@ lives are less than 20 minutes . Ytterbium also has 12 meta states , with the most stable being 169mYb ( t1 / 2 46 seconds ) . 
