The Ionic Crystal Lattice
Up to this point we have been considering what would happen if a single Li atom and a single H atom were combined. When a large number of atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. of each kind combine, the result is somewhat different.
Electrons are again transferred, and ions are formed, but the ions no longer pair off in twos. Instead, under the influence of their mutual attractions and repulsions, they collect together in much larger aggregates, eventually forming a three-dimensional array like that shown in the following figure. On the macroscopic level a crystalA solid with a regular polyhedral shape; for example, in sodium chloride (table salt) the crystal faces are all at 90° angles. A solid in which the atoms, molecules, or ions are arranged in a regular, repeating lattice structure. of solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. lithium hydride is formed.
The formation of such an ionic crystal latticeAn orderly, repeating arrangement of points in 3-D space in which each p;oint has surroundings identical to every other point. A crystal's constituent atoms, molecules, and ions are arranged about each lattice point. results in a lower potential energyA system's capacity to do work. than is possible if the ions only groupThose elements that comprise a single column of the periodic table. Also called family. into pairs. It is easy to see from the figure of the crystal lattice why this should be so. In an ion pair each Li+ ion is close to only one H– ion, whereas in the crystal lattice it is close to no less than six ions of opposite charge. Conversely each H– ion is surrounded by six Li+ ions. In the crystal lattice therefore, more opposite charges are brought closer together than is possible for separate ion pairs and the potential energy is lower by an additional 227 kJ mol–1. The arrangement of the ions in a crystal of LiH corresponds to the lowest possible energy. If there were an alternative geometrical arrangement bringing even more ions of opposite charge even closer together than that shown in the figure, the Li+ ions and H– ions would certainly adopt it.