What does the number written at the top of the ionic compound formula represent?

In the context of an ionic compound formula, the number written at the top often indicates the charge of the individual ions that make up the compound. For example, in the case of calcium chloride, the formula can be represented as CaCl₂. Here, the '+2' associated with calcium signifies its ionic charge, which indicates that each calcium ion (Ca²⁺) has lost two electrons, resulting in a charge of +2.

This charge is crucial because it informs you how many chloride ions (each with a charge of -1) are needed to balance the charge of the calcium ion to create a neutral compound overall. Thus, understanding the charge of individual ions helps in predicting the formulas and ratios in ionic compounds.

The other options do not accurately describe what the number at the top indicates. For instance, it does not represent the total number of ions in the compound, nor the charge of the compound as a whole. Additionally, it does not indicate the number of molecules, as ionic compounds typically do not dissolve in discrete molecule quantities like covalent compounds do. Therefore, recognizing that the number refers to the charge of the individual ions is integral to understanding ionic compound formation.