 The isotopes of ytterbium range in atomic weight from 147 @.@ <unk> atomic mass unit ( u ) for <unk> to 180 @.@ <unk> u for <unk> . The primary decay mode of ytterbium isotopes lighter than the most abundant stable isotope , 174Yb , is electron capture , and the primary decay mode for those heavier than 174Yb is beta decay . The primary decay products of ytterbium isotopes lighter than 174Yb are thulium isotopes , and the primary decay products of ytterbium isotopes with heavier than 174Yb are lutetium isotopes . 
