Metal oxides are not acidic. Most metal oxides are basic, and a few demonstrate traits of both acidic and basic compounds. Acidic oxides are formed by non-metals.
An oxide that can react with either an acid or a base is called an amphoteric oxide. These oxides are formed by metalloids, which are elements possessing properties of both metals and non-metals. The metals and non-metals closest to the metalloids on the periodic table are also capable of forming amphoteric oxides.
Aluminum, gallium, germanium, tin and lead are examples of metals that form amphoteric oxides. More reactive metals form oxides with no acidic properties. These metals include lithium, sodium, calcium and barium.