acidic oxide

Usually, non-metal oxides are acidic, such as NO2, SO2, OCl2; because those non-metals are electrophilic. When they are being oxidized, they will be in a situation of "lacking electron", and thus they will try to take electron from other molecules. Therefore, when non-metal oxides dissolve in water, it will take the electron from hydrogen atom, and turn the covalent-bonded hydrogen into hydrogen ion (H+).

Therefore, it is usually right to say "when an element has acidic oxide, the element is a non-metal". However, there are exceptional cases. For example, the metal oxide, Manganese (VII) oxide (Mn2O7), is acid, not basic.

Most metal oxide are insoluble in water or are alkalis.
e.g.oxides of Na, K, Ba & Ca are alkalis and give OH- ions when dissolving in water which is alkaline
　oxides of metal other than Na, K, Ba & Ca are insoluble in water
Thus, metal oxides would not be acidic oxide

If an element has acidic oxide, it means that its oxides will dissolve in water and form an acidic solution. This implies the element itself pulls in electrons. All metals like to give out electrons and thus, elements having acidic oxides will not be metal.