 Natural ytterbium is composed of seven stable isotopes : <unk> , <unk> , <unk> , <unk> , <unk> , 174Yb , and <unk> , 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 , <unk> with a half @-@ life of 4 @.@ 18 days , and <unk> 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 <unk> ( t1 / 2 46 seconds ) . 
