In order to find the conductivity ( p or n type ) in semiconductors such as V2O5 or CuBr i have to know if the oxidation state of the element is maximum or minimum. The d block elements have multiple oxidation states. (eg: Cr from +2 up to +6). But I should be able to find the most common ones which for Cr are +2 ( minimum ) +3 ( maximum ). Is there any general rule in order to distinguish the most common
oxidation states of transition elements?