Question 7
What is the chemical formula of aluminium oxide?
The right answer is the one marked above.
Al2O3. Aluminium has a valency of 3 and oxygen a valency of 2. Crossing over puts the 2 on the aluminium and the 3 on the oxygen.
Al3O2 has them the wrong way round — a common slip. The valency of one element becomes the subscript of the other, never its own.