 Contrary to most other lanthanides , which have a close @-@ packed hexagonal lattice , ytterbium crystallizes in the face @-@ centered cubic structure . As a result , its density ( 6 @.@ 973 g / cm3 ) is significantly lower than , e.g. , those of the neighboring elements thulium ( 9 @.@ 32 g / cm3 ) and lutetium ( 9 @.@ 841 g / cm3 ) . The melting and boiling points of ytterbium are also significantly lower than those of thulium and lutetium . These properties stem from the closed @-@ shell electron configuration of ytterbium ( [ Xe ] <unk> <unk> ) , which causes only the two 6s electrons to be available for metallic bonding ( in contrast to the other lanthanides where three electrons are available ) . 
