Final answer:
The formula unit for an ionic compound represents the simplest ratio of positive and negative ions that result in a neutral compound, typically involving metals and nonmetals. A molecular formula for a molecule indicates the actual number of atoms of each element in a molecule, formed by covalent bonds between nonmetals. The periodic table helps distinguish between the two.
Step-by-step explanation:
The difference between a formula unit for an ionic compound and a molecular formula for a molecule lies in the types of elements involved and the nature of their bonding. A formula unit refers to the simplest, neutral collection of ions in an ionic compound, which is comprised of metals and nonmetals. The periodic table can be used to determine which elements fall into these categories. On the other hand, a molecular formula represents the actual number and type of atoms in a molecule, which are bonded covalently and generally involve nonmetal elements.
For ionic compounds, the formula mass is used, which is the sum of atomic masses of all the elements in the empirical formula (simplest ratio of ions), with each multiplied by its corresponding subscript. This contrasts with a molecular compound's molecular mass, which is computed using the molecular formula, representing the total mass of all atoms in the actual molecule.
To summarize, the key distinction is that molecular formulas represent covalently-bonded nonmetals and give the specific numbers and types of atoms, whereas formula units represent ionic compounds composed of metals and nonmetals and indicate the simplest ratio of ions that results in neutrality.