Answer:
Chlorine and Electron Affinity
As fluorine sits atop chlorine in the periodic table, most people expect it to have the highest electron affinity, but this is not the case. Fluorine is a small atom with a small amount of space available in its 2p orbital. Because of this, any new electron trying to attach to fluorine experiences lower electron affinity from the electrons already living in the element's 2p orbital. Since chlorine's outermost orbital is a 3p orbital, there is more space, and the electrons in this orbital are inclined to share this space with an extra electron. Therefore, chlorine has a higher electron affinity than fluorine, and this orbital structure causes it to have the highest electron affinity of all of the elements.
Electron Affinity Trends
Atoms and molecules have certain qualities, one of which is electron affinity. As a rule of thumb, when you are looking at the periodic table, you can expect electron affinity to increase as you move from left to right across the periodic table -- excluding the noble gases -- and to decrease when you move down a group. Therefore, elements with the highest electron affinity in the top right corner of the periodic table. Chlorine, however, breaks this rule.
