Ionic solids are simple solids made of positive and negative ions held together by ionic bonds.
The structure of ionic solid crystals can vary depending on the charges and sizes of the ions, but there are a few basic “rules” that all ionic solids follow. The easiest example to look at is good-old table salt, sodium chloride. This is a simple example, since both ions are single charged atoms and the charges are the same producing a 1:1 ratio.
The ions in table salt are arranged in a pattern of + - + - + - in three dimensions. A crystal of sodium chloride would look like the diagram below (the larger blue spheres represent chloride ions and the smaller green spheres represent sodium ions).
If it is difficult to visualize this, or to understand what this is showing, it may help to realize that this crystal is made of the thee layers below, one on top of the other.
In each layer, the ions are aranged + - + - both vertically and horizontally. But, of course, this is actually a three-dimensional structure.
As a result of the three-dimensional arrangement each chloride ion in this structure doesn't just have one sodium ion next to it. In fact, a chloride ion inside the crystal has 6 sodium ions around it (front, back, left, right, top and bottom). The same, of course, is true for a sodium ion surrounded by chlorides.
This arrangement means two things. First, there is a great deal of attraction for each ion, resulting in a crystal that resists melting. Thus, ionic compounds have high melting points. Secondly, there are no real molecules of an ionic solid. The formula (in this case NaCl) does not describe a single unit, but rather just the ratio of positive to negative ions. This is the reason that ionic formulas are always written in empirical form (smallest whole number ratio) and that chemical stock bottles list a formula weight rather than a molecular weight.