In the chemical process called electron transfer, an electron is transferred from one atom or molecule to another. (We will talk about electron transfer extensively in Chapter 20.) A simple electron transfer reaction is $$ \mathrm{A}(g)+\mathrm{A}(g) \longrightarrow \mathrm{A}^{+}(g)+\mathrm{A}^{-}(g) $$ In terms of the ionization energy and electron affinity of atom A, what is the energy change for this reaction? For a representative nonmetal such as chlorine, is this process exothermic? For a representative metal such as sodium, is this process exothermic? [Sections 7.4 and 7.5\(]\)

We can write the energy change for the given reaction as follows: $$ \Delta E = -\mathrm{IE} + \mathrm{EA} $$ Where: - ΔE = energy change for the reaction - IE = ionization energy of atom A - EA = electron affinity of atom A Expressing the energy change of the electron transfer in terms of ionization energy and electron affinity is crucial to determine if the process is exothermic or not.

For chlorine (a nonmetal), we must find the values for ionization energy (IE) and electron affinity (EA) and apply the formula from Step 1 to calculate the energy change of the reaction. The ionization energy for chlorine is 1,251 kJ/mol, and its electron affinity is 349 kJ/mol. Plugging these values into the formula, we get: $$ \Delta E_\mathrm{Cl} = -1,251\,\text{kJ/mol} + 349\,\text{kJ/mol} = -902\,\text{kJ/mol} $$ Since the energy change is negative, the electron transfer process between two chlorine atoms is exothermic.

Similarly, for sodium (a metal), we must find the values for ionization energy (IE) and electron affinity (EA) and apply the formula from Step 1 to calculate the energy change of the reaction. The ionization energy for sodium is 496 kJ/mol, and its electron affinity is approximately 53 kJ/mol. Plugging these values into the formula, we get: $$ \Delta E_\mathrm{Na} = -496\,\text{kJ/mol} + 53\,\text{kJ/mol} = -443\,\text{kJ/mol} $$ Again, since the energy change is negative, the electron transfer process between two sodium atoms is also exothermic. In conclusion, the energy change for the process of electron transfer can be expressed in terms of ionization energy and electron affinity as: $$ \Delta E = -\mathrm{IE} + \mathrm{EA} $$ The process is exothermic for both chlorine (nonmetal) and sodium (metal) as the energy changes are negative.