Chemistry: The Central Science (13th Edition)

$Cr_2O_3$ $Cr$ is chromium. $O$ is oxygen, which his called oxide when naming compounds. Oxygen has a charge of -2. The compound is neutral, so the charge/oxidation number of the manganese element was +3. (The 3 oxygen atoms contributed a -6 charge, which must be offset by a +6 charge from the chromium. There are 2 chromium atoms, so +3 each.) The name is written as element(oxidation number) element so $Cr_2O_3$ is chromium (III) oxide.