Pronunciation: (mang'gu-nēs", -nēz"),[key] —n.Chem. a hard, brittle, grayish-white, metallic element, an oxide of which, MnO2(man'ganese diox'ide), is a valuable oxidizing agent: used chiefly as an alloying agent in steel to give it toughness. Symbol: Mn; at. wt.: 54.938; at. no.: 25; sp. gr.: 7.2 at 20°C.