a toxic metallic element that exists in two allotropic forms and occurs principally in stibnite. The stable form is a brittle silvery-white crystalline metal that is added to alloys to increase their strength and hardness and is used in semiconductors. Symbol: Sb; atomic no: 51; atomic wt: 121.757; valency: 0, –3, +3, or +5; relative density: 6.691; melting pt: 630.76°C; boiling pt: 1587°C