Ionic Structures
Will united ions be defeated? Lattice energy
The structures of ionic compounds are very varied. The positive and negative ions are kept as close as possible. Negative ions are surrounded by the maximum number of positive ions and vice versa. The number of ions and their size determine the final structure of the crystal. Here are some examples.
Can you tell which atoms act as positive ions and which as negative ions?
In an ionic structure, ions of opposite signs attract very strongly. That this force is very great is shown by the melting points of the ionic salts. For example, to melt common salt (NaCl) and convert it to a liquid state, about 800 °C is needed. This means a high amount of energy. The energy that must be supplied to an ionic compound to overcome the forces between the ions and be able to melt it is called lattice energy.
The graph shows the melting points of the salts formed between Lithium and the elements of the penultimate row (halogens) of the periodic system. You can see that they are all very high but not all the same. Why?
Imagine that you want to pack balls of two different sizes. The best packing will occur when the size of the balls is similar. As the ratio between the radii of the positive ion (r+) and the negative ion (r-) are more similar (r+/r- closer to 1), the packing is better, the lattice energy is higher, and the temperature necessary to melt is higher.