Ch.8 - Periodic Properties of the ElementsWorksheetSee all chapters
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Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
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Ch.4 - Chemical Quantities & Aqueous Reactions
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Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds
Trends in Electron Affinity

Concept #1: Understanding Electron Affinity

Transcript

In this new video, we're going to talk about electron affinity.
We're going to say electron affinity is the energy change in kilojoules or in joules from the addition of one mole of electrons to one mole of gaseous atoms or ions. We're going to consider that electron affinity is the complete opposite of ionization energy. Ionization energy is the energy it takes to rip off an electron. Electron affinity is the opposite, it's the energy that's required to place an electron onto something.
We're going to say we're adding an electron to something, so we're creating new bonds. Because you're creating a bond between the atom and this new incoming electron. If you're creating that, then it's an exothermic reaction, if we're talking about heat, or it's an exergonic reaction, if we're dealing with just energy. Heat/energy, same thing. If we're talking about heat, thermal heat, then it's exothermic. If we're just talking about regular energy, it's exergonic.
In both cases, when we're talking about exergonic or exothermic, the change in energy was going to be less than zero. Making bonds is a negative process. We're releasing energy in order to create bonds.
We're going to say generally speaking, for electron affinity, we're going to say it increases going from left to right. Then we're going to say it decreases going down a group. So going from left to right across a period it increases and it decreases going down a group.
Here we have a gaseous compound, could be fluorine gas and it basically absorbs or takes in that electron. We're creating a bond. It's an exergonic process. That's why we expect our change in energy to be negative because we expect it to be less than zero because we're forming a bond.

Electron affinity is the energy required to add 1 mole of electrons to a gaseous element. 

Example #1: Rank the following elements in order of increasing electron affinity. 

Cs, Hg, F, S

Se, S, Si

Practice: Shown below are the numerical values for electron affinities (EA's). Match the numerical values with each of the following elements provided in the boxes.