* Ionic Compounds: K2Cr2O7 is an ionic compound. This means it's formed by the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions).

* Potassium (K+) is the cation.

* Dichromate (Cr2O7^2-) is the anion.

* Formula Units: Ionic compounds don't exist as discrete molecules. Instead, they form a crystal lattice where the ions are arranged in a repeating pattern. The formula unit represents the simplest whole-number ratio of ions in the compound.

Therefore, K2Cr2O7 is a formula unit, not a molecule.