An Ionic Crystal. Figure P23.57 shows eight point charges arranged at the corners of...

An Ionic Crystal. Figure P23.57 shows eight point charges arranged at the corners of a cube with sides of length d . The values of the charges are + q and -q, as shown. This is a model of one cell of a cubic ionic crystal. In sodium chloride (NaCl), for instance, the positive ions are Na⁺ and the negative ions are Cl^-. (a) Calculate the potential energy U of this arrangement. (Take as zero the potential energy of the eight charges when they are infinitely far apart.) (b) In part (a), you should have found that U < 0. Explain the relationship between this result and the observation that such ionic crystals exist in nature.