How is arsenic trioxide amphoteric?

[Chem_Mod]

For problem 10.19, how do we know that the sulfur trioxide is acidic and that arsenic trioxide is amphoteric? Looking at the Lewis structure of SO3, it seems that there are no places for the molecule it accept an electron pair (Lewis Acid).

[Chem_Mod]

There are general trends that one should try to keep in mind when determining acidity/basicity of compounds. One of these trends is that metal oxides (Group I and Group II oxides) are generally basic, nonmetal oxides (Group 16 and 17 oxides) are generally acidic, and things in between can generally be amphoteric.