Q. Consider the following table, showing properties of the neutral atoms and most-stable ions of four representatives elements from the fourth period.
| Atom | Ionization Energy
(kJmol) | Electron Affinity
(kJmol) | Atomic Radius (pm) | Most Stable Ion | Ionic Radius (pm) |
| K | 419 | N/A | 227 | K+ | 133 |
| Ca | (1)589
(2)1145 | N/A | 197 | Ca2+ | 99 |
| S | N/A | (1)−201
(2)+532 | 104 | S2− | 184 |
| Cl | N/A | −348 | 99 | Cl− | 181 |
The Born-Haber cycle is a way to describe the formation of a binary ionic compound, starting with a metal and a nonmetal, both in their elementary form. For the case in which an alkali metal reacts with a diatomic halogen. (e.g., potassium and chlorine), such a reaction may be represented by a generic equation of the form:
M(s)+12X1→M+X−(s)
The reaction is envisioned an occurring as a series of steps:
(1) The metal is converted to gaseous atoms;
(2) The gaseous metal atoms are converted to gaseous cations;
(3) The nonmetal is converted to gaseous atoms;
(4) The gaseous nonmetal is converted to gaseous anions; and
(5) The gaseous cations and anions unite to form a crystalline ionic solid.
As you see, ionization energies are provided for the metals, but not the nonmetals.
Similarly, electron affinities are provided only for the nonmetals, not for the metals.
Based on the information given in the table, and your knowledge of the energies involved, answer the following.
Of all the electron affinities shown, only one is positive. Why?
