What is the formula of aluminum oxide?
In this case, the formula that the question is looking for is the number of atoms of each type of element present in the compound. For instance, the chemical formula for methane is CH₄, one carbon atom and four hydrogen atoms. Let’s start by breaking up the name aluminum oxide into two parts, aluminum and oxide. We can use the principles of the naming of ionic compounds to work out what the formula of aluminum oxide is. In the name of ionic compounds, the bit that describes the cation comes first. And the bit that describes the anion comes second.
For the cation portion, if the ion is monatomic, then we simply use the name of the element. In this case, the name of our cation is aluminium. Aluminium has the symbol Al and as it is in group 13 of the periodic table forms ions with a three plus charge. The second part of an ionic compound name is the anion. If the anion is monatomic, then the anion portion of the name is the root of the element name plus “ide”. Here, the phrase monatomic simply means that the ion is derived from one atom. In this compound, our negative ion, the anion, is called oxide. The ide portion means that the ion is monatomic. And the ox portion refers to oxygen.
If you refer to your periodic table, you’ll see that the symbol for oxygen is O and that oxygen will form two minus ions because it is in group 16. Ionic compounds must be neutral overall. So we need two equivalents of aluminium three plus and three equivalents of oxygen two minus to make a neutral compound. We can check that it’s neutral overall by doing two times three plus three times minus two. This is equal to six minus six which is equal to zero. So the formula of aluminum oxide is Al₂O₃.