The valence electrons are the electrons in the last shell or energy level of an atom. They do show a repeating or periodic pattern. The valence electrons increase in number as you go across a period. Then when you start the new period, the number drops back down to one and starts increasing again.
A quick way to determine the number of valence electrons for a representative element is to look at which group is it in. Elements in group Ia have 1 valence electron. Elements in group IIa have 2 valence electrons. Can you guess how many valence electrons elements in group VIa have? If you guessed 6 valence electrons, then you are correct! The only group of representative elements that this method doesn't work for is group 0. Those elements certainly have more than 0 valence electrons; in fact, all of them except for helium have 8 valence electrons. Why doesn't helium have 8 valence electrons? Think for a moment about how many electrons helium has - it has a total of only two electrons, so helium only has 2 valence electrons.
So generally speaking, the number of valence electrons stays the same as you go up or down a group, but they increase as you go from left to right across the periodic table. The preceding statement works very well for the representative elements, but it comes a bit short of the truth when you start talking about the transition elements.
Electrons going into the d sublevels of the transition metals complicate this pattern. In some ways these electrons behave like valence electrons. In some other ways they behave like shielding electrons, which are discussed in the next section. The first electrons into a d sublevel seem to behave more like valence electrons but the last ones seem to act more like shielding electrons, with variations along the way. Switching the order from 4s3d to 3d4s is one way to represent this.
As it turns out, the idea of valence electrons is not very useful for transition metals, at least not in a reliable, predictable way.
Electron Dot Diagrams
For a chemist, the valence electrons are quite possibly the most important electrons an atom has. "Why the valence electrons?", you might ask. Well, since the valence electrons are the electrons in the highest energy level, they are the most exposed of all the electrons ... and, consequently, they are the electrons that get most involved in chemical reactions. Chemists use a notation called electron dot diagrams, also known as Lewis diagrams, to show how many valence electrons a particular element has. An electron dot diagram consists of the element's symbol surrounded by dots that represent the valence electrons. Typically the dots are drawn as if there is a square surrounding the element symbol with up to two dots per side. (An element will never have more than eight valence electrons.)
As we discussed above, you can determine how many valence electrons an element has by determining which group it is in. What would the dot diagram for helium look like? It has 2 valence electrons, so it should have 2 dots like this: .He. or He:
Example 6 in your workbook has a few more dot diagrams to study, then try your hand at the ones in Example 7.
Answers for Example 7:
K is in group Ia, so it has 1 valence electron (1 dot).
Al is in group IIIa, so it has 3 valence electrons (dots).
As is in group Va, so it has 5 valence electrons (dots).
F is in group VIIa, so it has 7 valence electrons (dots).
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