VALENCE ELECTRONS

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.

For example, when you go across the table from carbon to nitrogen to oxygen, the number of valence electrons increases from 4 to 5 to 6. As we go from fluorine to neon to sodium, the number of valence electrons increases from 7 to 8 and then drops down to 1 when we start the new period with sodium. Within a group--starting with carbon and going down to silicon and germanium--the number of valence electrons stays the same.

C 4 N 5 O 6 F 7 Ne 8 Na 1 Si 4 Ge 4

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.

	Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn
outer configuration	4s23d1	4s23d2	4s23d3	4s13d5	4s23d5	4s2 3d6	4s2 3d7	4s2 3d8	3d104s1	3d104s2
apparent valence electrons	3	2-4	2-5	2-6	2-7	2 or 3	2 or 3	2 or 3	1 or 2	2

As it turns out, the idea of valence electrons is not very useful for transition metals, at least not in a reliable, predictable way.