Here is the answer and explanation of question number 24 of our midterm review:
24) Which element on the periodic table should have the most favorable electron affinity? Which element would have the most negative electron affinity value? Explain your choices.
Essentially, these two questions are asking the same thing. The element that has the most favorable electron affinity is actually the element that has the most negative electron affinity value. The electron affinity value measures how much energy is released when an atom gains an electron or the amount of energy required for an atom to gain an electron. The more negative the electron affinity value of the element is (the more favorable electron affinity), the greater the amount of energy an atom releases when it gains an electron. Positive electron affinity values signify that for the atom to gain an electron, energy must be added to the atom (this arrangement is very unstable and when the application of energy is ceased, the atom will expel the electron and return to a lower energy level).
The image above is a 3D graph of the electron affinity values of the various elements of the periodic table. As can be seen, chlorine has the most favorable electron affinity, just a bit more favorable than fluorine, the electronegative element.
The reason that chlorine (Cl) is the element with the highest electron affinity (the most negative value) is very similar to the reason that fluorine is the most electronegative element. As it is in Group 17, chlorine is longing to obtain the electron configuration of the noble gas argon by gaining one electron and becoming a 1+ cation. It has a strong attraction to electrons due to its strong positive charge in its nucleus (compared to the other elements in its period). Furthermore, as chlorine is relatively high in the periodic table (it is in one of the lesser periods), there are fewer orbital “shells” shielding free electrons from the positive attraction of the nucleus. Thus, a large amount of energy is released when chlorine gains an electron.
Interestingly, the reason that chlorine has a larger electron affinity than fluorine, even though fluorine is the most electronegative, is due to the fact that fluorine is a smaller atom than chlorine. When an electron is added to fluorine, there is a greater repulsion between the electrons in the orbitals than there is with chlorine. Thus, even though fluorine might be more electronegative than chlorine, chlorine has a more favorable electron affinity.
Electron Affinity 3D Graph: http://www.webelements.com/periodicity/electron_affinity/cityscape_chart.html