Here's why:

* Molecular Structure: Propane (C3H8) has three carbon atoms, while butane (C4H10) has four. This difference in carbon atoms means butane has a higher molecular weight.

* Molecular Weight: Propane has a molecular weight of 44.1 g/mol, while butane has a molecular weight of 58.12 g/mol.

Therefore, butane is heavier than propane.