Electron+Affinity

=Electron Affinity=

What is Electron Affinity?
Electron Affinity is the energy released when a neutral atom gains an electron to form a negatively charged ion. Atom + e **-** --> Atom **-** + Energy

Electron Affinity can also be reversed so that it can be the amount of energy required to break an electron off from a negatively charged ion. Atom **-** + Energy --> Atom + e **- ** (Click picture to expand and see a bigger version)

 Period Trends
When moving across a period, the general rule for electron affinity is that is become more and more negative within the //p// sublevel. However, there is an exception to this rule. This occurs among groups 14 and 15. When an element from groups 14 gain an electron, they gain a half-full sublevel. This makes them stable. In groups 15, they already have a half-full sublevel so they do not want to gain or lose any electrons.

Group Trends
Group trends occur less regularly than period trends. The general rule for group trends is electrons add with greater difficulty down a group. This result is caused by two factors. The first is a very slight increase in nuclear charge, which in effect, increases electron affinities. The second is the increase in size of the atomic radius, which decreases electron affinity. Overall, the size of the radius dominates of the nuclear charge. But of course, as with every rule, there is an exception. The heavy transition metals are usually the same size or even smaller as they go down a group.

Adding Electrons to Negative Ions
It is VERY hard to add an electron to an ion that already has a negative charge. It can also be very hard to add electrons to atoms that are already stable. Because of this, Electron Affinities are always positive.

**Electron Affinities and Electron Configurations**
Since the stability of an electron is a major factor in electron affinity, the two properties are closely related. If an atom has a stable configuration, then it has a very low electron affinity. This is because it takes a lot of energy to force them to change their electron configurations. For example, Hydrogen(1s^1) has an electron affinity of 72.8 kJ/mol while Helium(1s^2) has a very low electron affinity. The difference is because Helium has a full energy level while Hydrogen only has a half full energy level. In general, elements with full energy levels almost always have a very small electron affinity. Half-full energy levels only sometimes have a small electron affinity.

Fun Visuals!
[|Click here]for a fun visual. You may have to scroll down the page a bit


 * Element || Electron Affinity (kJ/mol) || Electron Configuration ||
 * H || 72.8 || 1s^1 ||
 * He || <0 || 1s^2 ||
 * Li || 59.8 || [He]2s^1 ||
 * Be || <0 || [He]2s^2 ||
 * B || 27 || [He]2s^2 2p^1 ||
 * C || 122.3 || [He]2s^2 2p^2 ||
 * N || <0 || [He]2s^2 2p^6 ||
 * O || 141.1 || [He]2s^2 2p^4 ||
 * F || 328 || [He]2s^2 2p^5 ||
 * Ne || <0 || [He]2s^2 2p^6 ||

Click to see a bigger version

Knowledge Questions:

 * 1) What is the definition of electron affinity?
 * 2) What are the two groups where there is an exception to the general rule for period trends?
 * 3) Is it easy to add an electron to an ion with a negative charge?

Critical Thinking:

 * 1) Are electron affinities positive or negative? Why?

Calculation Problem:

 * 1) What is the electron affinity of noble gases?

Link to Powerpoint Presentation
http://docs.google.com/Present?docid=dd3cj99s_7cz2hsxch&invite=cxtqds2  