Here's why:

* Crystalline solids have a highly ordered, repeating arrangement of atoms or molecules. This arrangement gives them specific geometric shapes and predictable properties.

* Amorphous solids lack this regular structure. Their atoms are arranged randomly.

Iodine molecules (I2) arrange themselves in a layered lattice structure where the molecules are held together by van der Waals forces. This regular arrangement makes iodine a crystalline solid.

You can even see this crystalline structure in the form of shiny, dark-purple crystals of iodine.