Question

How do ionization energy and electron affinity determine whether atoms of elements will combine to form ionic compounds?

Answer 1

To quote my trusty copy of Whitten's General Chemistry, Third Edition: "ionic bonding occurs most easily when elements that have low ionisation energies (metals) react with elements having high electronegativities and high electron affinities (nonmetals)."

Explanation:

Let us consider the definitions of ionisation energy and electron affinity for a moment.

• (First) ionisation energy - The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom to form an ion with a `"1+"` charge.

• Electron affinity - The amount of energy absorbed when an electron is added to an isolated gaseous atom to form an ion with a `"1-"` charge.

Note, then, that these two values (which are constants for each element) are NOT simply reverses of each other :

• `"EF"_1"( reversed ) : ""X" ^+(g) + e^(-) -> "X"(g)`

• `"EA: " "X"(g) + e^(-) -> "X" ^(-) (g)`

This demonstrates that first ionisation energy determines the tendency of an atom to loose its most loosely bound electron to become a `"1+"` ion; on the other hand, electron affinity is the tendency for electrons to be taken in by an atom, and for that atom to become a `"1-"` ion. The greatest stability is achieved when electron affinity is negative, since this would refer to the atom liberating energy when the electron is added: things are most stable when they harbour the least energy possible.

Thus, a low first ionisation energy means that very little energy is required to remove that electron from the atom, and so means that the atom forms its `"1+"` cation quite readily. A high electron affinity, where "high" refers only to the magnitude of the number, and not its sign, means that the atom is more inclined to taken in an electron to become a `"1-"` anion.

Thus, it is now easy to see how ionisation energy and electron affinity affect the likelihood of ionic bonding taking place, by having an impact on the tendency of cation and anion formation.