Question 19
Across period 3, sodium oxide is strongly basic and chlorine oxide is strongly acidic. What is aluminium oxide?
The right answer is the one marked above.
Amphoteric. Al2O3 shows both acidic and basic properties, and oxides like that are called amphoteric oxides.
Notice where it falls in the row: exactly at the turn between the basic side and the acidic side. Going from left to right along a period, the oxides become less basic and more acidic — Na2O strongly basic, MgO weakly basic, then Al2O3, and on through SiO2 to Cl2O7.