Why are metal oxides bases?
In both metal and nonmetal oxides, hydration produces E-O-H, which can either be an oxoacid or a hydroxide base. The difference arises from the identity of the bond that is most ionic, E-O or O-H.
Base: E-O-H(aq) ? E+(aq) + OH-(aq)
Acid: E-O-H(aq) ? EO-(aq) + H+(aq)
The greatest electronegativity difference (E-O or O-H) determines which is the most ionic. Dc of O-H is 1.4; if Dc of E-O is greater than 1.4, we have a base. This is found for metals.