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Electron affinity is defined as the energy released when you add an e- to a gas phase atom to form a -1 ion. Elements at the top right have the highest electron affinity because they release the most energy to form that -1 ion. One explanation is that they become most stable when they gain that e-. For example, fluorine is a halogen, and if it gains an e-, it gains a noble gas configuration, which is very stable and low in energy. Therefore, it has the highest electron affinity because it becomes most stable when it gains that electron.
Most nonmetals have a higher electron affinity since they have more valence electrons than metals and can easily attract more electrons and fulfill their octet. Also, nonmetals' valence electron shell is closer to the nucleus thus making it harder to remove electrons and easier for those elements to attract/remove an electron from another element (most likely a metal).
The further right you go, the more electro-negative an element is and the further up you go the more electro-negative an element becomes. Elements in the top right corner are the most electro-negative while those in the bottom left are the least electro-negative. Hope that helps..
Something that is also interesting is that Chlorine actually has a higher electron affinity than Fluorine. Fluorine is smaller than Chlorine, therefore there is less space availalbe in its 2p orbital. Chlorine's outer orbital is a 3p, so therefore there's more space for electrons to be shared in this space with one another.
As you travel up and towards the right across the periodic table, elements become more and more electronegative, therefore have a higher electron affinity (attractiveness). This is because as you go right, there's an increase in valence electrons, and electrons are more attracted to valence shells that are more full. Also, as you go up, atoms get smaller and n (the number of shells/outermost shell) gets smaller, they tend to have a higher electron affinity too!
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