Chapter 3, Problem 87AP

At large interatomic separations, an alkali halide molecule MX has a lower energy as two neutral atoms, $\text{M}+\text{X}$ ; at short separations, the ionic form $\left(\text{M}+\right)\left(\text{X}-\right)$ has a lower energy. At a certain distance, $R_c$ , the energies of the two forms become equal, and it is near this distance that the electron will jump from the metal to the halogen atom during a collision. Because the forces between neutral atoms are weak at large distances, a reasonably good approximation can be made by ignoring any variation in potential V(R) for the neutral atoms between $R_c$ and $R=\infty$ . For the ions in this distance range, $R_c$ is dominated by their Coulomb attraction.

(a) Express $R_c$ for the first ionization energy of the metal M and the electron affinity of the halogen X.

(b) Calculate $R_c$ for LiF, KBr, and NaCl using data fromAppendix F.