Question

Ionic crystals are often transparent, whereas metallic crystals are always opaque. Why?

Short answer

The free electrons reflect back the light that passes through the material, which makes these type of crystals to appear opaque or shiny

Step-by-step solution

 Step 1: Determine metalic crystal are always opaque

We begin with differentiating the structure of metallic crystals and ionic crystals- Metals are basically formed from metallic bonds and metals have a sea of electrons surrounding around the atoms of the material These free electrons causes that light passes through the material to be reflected back, hence metallic crystals are opaque- For the ionic crystals, the opposite happens- Ionic crystals do not have those free electrons because these type of crystals are insulator. In this sense, only a small amount of the incident light is reflected back, most are being absorbed or passed through the material- Hence, ionic crystals are often transparent.

Therefore

Metallic crystals have sea of electrons surrounding the atoms of the material These free electrons reflect back the light that passes through the material, which makes these type of crystals to appear opaque or shiny. Ionic crystals, on the other hand, do not have those free electrons-

Hence, only small amount of light is reflected back and most are being absorbed or passed through the material These make ionic crystals to appear to be often transparent -