-The more metallic an element, the more basic is its oxide.
-The more non-metallic an element, the more acidic is its oxide.
Some common acidic oxides are: CO2, NO2, N2O3, N2o5, SO2, SO3, P2O3, P2o5, Cl2O
The acidic nature of an oxide decreases down group 5. In group 6 oxygen is neutral but sulfur (SO2 and SO3) is acidic. In group 7 all oxides execpt F2O are acidic, and the oxides of chlorine (Cl2) and Cl207) dissolve in water to form strong perchloric acid (HCLO4) which acts as an oxidant.
-Aluminium forms amphoteric oxides (as do zinc, lead, tin)*
*not in syllabus - just a general note