Ionic crystals are solid structures formed by the electrostatic attraction between positively and negatively charged ions. These crystals typically have a regular, repeating arrangement, which gives them a distinct geometric shape. Common examples of ionic crystals include sodium chloride (table salt) and magnesium oxide. The strong forces holding the ions together result in high melting and boiling points, making ionic crystals generally hard and brittle. When dissolved in water or melted, they can conduct electricity, as the ions become free to move. This property is essential in various applications, including electrolytes in batteries.