Final answer:
The formula for lithium oxide is Li2O, derived from combining two lithium ions (Li+) with one oxygen ion (O2-) to create an electrically neutral compound.
Step-by-step explanation:
The formula for lithium oxide, which consists of 2 lithium ions (Li+) and 1 oxygen ion (O2-), is Li2O. When writing the formula for an ionic compound, such as lithium oxide, we must ensure the compound is electrically neutral. Thus, two lithium ions each with a charge of +1 will balance out the charge of one oxide ion with a charge of -2. This approach is similar to the crisscross method, where the charges of the ions are used as subscripts for the opposite ion after dropping the signs.
When forming compounds with polyatomic ions, the same principle applies: the total positive charge must balance the total negative charge. For example, if combining lithium ions with the peroxide ion (O22-), the formula of the resulting ionic compound would be Li2O2.