a soft malleable silvery element of the lanthanide series of metals that occurs in monazite and is used to improve the mechanical properties of steel. Symbol: Yb; atomic no: 70; atomic wt: 173.04; valency: 2 or 3; relative density: 6.903 (alpha), 6.966 (beta); melting pt: 819°C; boiling pt: 1196°C