~Final answer:
Prefixes are omitted in naming ionic compounds because the composition is implied by the charges of the cation and anion which are generally known by chemists, while prefixes in molecular compounds denote the specific number of atoms due to varying compound ratios.
Step-by-step explanation:
Prefixes are not needed in naming ionic compounds because the formula unit sufficiently conveys the stoichiometry of the compound. In ionic compounds, the total positive and negative charges are balanced, and each ion typically has a common valence state that a chemist recognizes. For instance, NaCl is known as sodium chloride, and CaCl2 is calcium chloride, without the need for prefixes to indicate the number of ions. However, if a metal ion can exist in more than one charge state, like iron, a Roman numeral is used to specify the oxidation state, as in iron(II) chloride for FeCl2 and iron(III) chloride for FeCl3. Molecular compounds, unlike ionic compounds, do require prefixes to indicate the number of atoms because they can consist of the same elements in different ratios, such as SF6 for sulfur hexafluoride and N2O4 for dinitrogen tetroxide.