We can use information on the charges of the ions and their location in the lat-tice to calculate the energy of the system. Taking NaCl as an example (see figure12.9), focus on a Na+ ion in the center of the unit cell. We see it is surrounded by,and attracted to, six CI- ions. Just a bit farther away from the Na+ ion, however,there are 12 Na+ ions along the edges of the cubes, and there is a force of repulsionbetween the center Na+ and these ions. And if we still focus on the Na+ ion in the "center" of the unit cell, we see there are eight more Cl-ions at the corners of the cube, and these are attracted to the center Na+ ion. Taking into account all of the interactions between the ions in a lattice, we can calculate the lattice energy, Lattic Uthe energy of formation of one mole of a solid crystalline ionic compound when ionsin the gas phase combine (Table 12.1). For sodium chloride, this reaction is