Question

Is the formula we write for an ionic compound the molecular formula or the empirical formula? Why?

Short answer

The formula we write for an ionic compound is the empirical formula since it represents the simplest whole number ratio of ions present in the compound. This is due to the fact that ionic compounds don't have distinct molecules as in covalent compounds and need a neutral net charge by achieving a balance of positive and negative ions in the simplest ratio possible.

Step-by-step solution

Definition of Molecular and Empirical Formulas

A molecular formula shows the exact number of atoms of each element present in a molecule of the compound, while an empirical formula shows the simplest whole number ratio of the atoms in the compound. For example, the molecular formula for glucose is C6H12O6 which shows that one molecule has 6 carbon, 12 hydrogen and 6 oxygen atoms. Its empirical formula would be CH2O, which is the simplest whole number ratio of C:H:O as 1:2:1.

Ionic Compounds and Formulas

Ionic compounds consist of positively and negatively charged ions held together by electrostatic force. They don't form distinct molecules like covalent compounds. Instead, ions arrange themselves in a crystal lattice structure. As there are no discrete molecules in ionic compounds, we cannot write a molecular formula for them.

Empirical Formulas for Ionic Compounds

Since ionic compounds don't have discrete molecules, the formula we write for them represents the simplest whole number ratio of ions present. This reflects the basic principle of charge neutrality, where the net charge of the compound is zero. For example, the ionic compound sodium chloride (common table salt) consists of sodium ions (Na+) and chloride ions (Cl-) in a 1:1 ratio. The formula NaCl represents the simplest whole number ratio of the ions, which is an empirical formula.

Conclusion

The formula we write for an ionic compound is the empirical formula, as it represents the simplest whole number ratio of ions present in the compound. This is because ionic compounds don't have distinct molecules like covalent compounds, and the net charge of the compound must be neutral, thus requiring a balance of positive and negative ions in the simplest ratio possible.