 The isotopes of ytterbium range in atomic weight from 147 @.@ 9674 atomic mass unit ( u ) for 148Yb to 180 @.@ 9562 u for 181Yb . 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 . 
