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Why do solid ionic crystals conduct electricity poorly while melts and solutions can conduct it?

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A rough, translucent yellow crystal of elemental sulfur displayed against a solid black background. — crystal A crystal form of elemental sulfur. Sulfur has an atomic weight of about 32.06 atomic mass units (amu), which means that its molar mass is 32.06 grams.

Solid ionic crystals are poor conductors of electricity because their charged ions are locked into a rigid, ordered lattice structure, which prevents the ions from moving freely to carry an electric current. Additionally, in these solids, all electrons are typically held in filled shells, meaning there are no free electrons available to facilitate conduction.123

In contrast, when an ionic substance is melted or dissolved in a suitable solvent, the rigid lattice is broken down. This process allows the constituent ions to dissociate and move independently throughout the liquid. When an electric potential is applied to these melts or solutions, the now-mobile positive and negative ions can migrate toward the electrodes of opposite polarity, effectively conducting electricity through the fluid.4

While some solid crystals can exhibit a very small amount of ionic conductivity through a process called "ion hopping"—where ions move into vacant lattice sites—this is generally negligible compared to the conductivity observed in liquid electrolytes.54