Question

At large interatomic separations, an alkali halide molecule MX has a lower energy as two neutral atoms, M + X; at short separations, the ionic form(M+)(X-) 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 = infinity. For the ions in this distance range, V(R) is dominated by their Coulomb attraction.

Express Rc for the first ionization energy of the metal M and the electron affinity of the halogen X.

Answer 1

The energy of an alkali halide molecule is lower when it consists of neutral atoms at larger separations but becomes more stable as an ionic compound at shorter distances. This transition is associated with the lattice energy, which contributes to physical properties like the crystal's hardness and melting point.

Step-by-step explanation:

The energy considerations for an alkali halide molecule transitioning from neutral atoms (M + X) to ionic form (M+)(X-) revolve around the concept of lattice energy and the transition state at which the electron is transferred from the metal to the halogen atom. The lattice energy is influenced by factors such as the charges of the ions, the internuclear distances, and the ionic radii. The study of these energy changes provides insight into the properties of the materials, including hardness, melting points, electrical conductivity, and solubility in water.

At large atomic separations, the molecule has lower energy as two neutral atoms (M + X), while at short separations, the ionic form (M+)(X-) is more energetically favorable. The equilibrium bond length is the distance where the molecule has the lowest energy state. The lattice energy trends observed for alkali halides demonstrate that smaller, highly charged ions have higher lattice energies, which is integral to understanding ionic crystals and their characteristics.