Why is the product of the reaction "Bi + O2" Bi2O3 instead of Bi2O5? I am stuck on a homework question that states:


  A piece of solid bismuth is heated strongly in oxygen.


The answer key states the answer as $\ce{Bi + O2 → Bi2O3}$. However, it is to my understanding that the metal Bismuth has a charge of +5 because of its location on the periodic table. Therefore, $\ce{(+5)[charge of Bi]*2 + (-2)[charge of oxygen]*5 = 0}$ giving me $\ce{Bi2O5}$. Please advise.
Many elements have multiple valencies. The most common oxidation state of Bi is +3. Bismuth (V) is also possible. Read more here on Bismuth oxides.