Predicting the Number of Valence Electrons

Valence Electrons are the electrons in the outermost (energy level) open electron shell of an atom.
They are the electrons that can take part in chemical reactions, also called the "reactable electrons".
Some things you may need to know:
Open Shell: A shell that contains less than its maximum number of electrons
Closed Shell: A shell that contains exactly its maximum number of electrons.

All Noble Gases have 0 valence electrons, because they all have something called a stable octet.
A stable octet is when the shell has exactly 8 electrons on it.

example: Neon.
Atomic number: 10.
We all think back to grade 10, and remember the order of the number of electrons each shell can hold.
First shell holds 2 electrons.
Second shell holds 8.
Third shell holds 8.
Fourth (and final) shell holds 16.

As you can see, there is a perfect shell - or a stable octet. There is no room for any more electrons.

Valence electrons are all the electrons in an atom except those in the core, or in the filled d- or f- subshells.
A quick way to determine the number of valence electrons for a representative element is to look at which group is it in. 

As you can see in this periodic table, I have drawn on (messily) the group numbers. 

In group 1, there is 1 valence electron for each element. For group 2, there are 2 valence electrons for each element. And so on, until group 8, where there are no valence electrons, because Noble Gases ALWAYS have a stable octet.

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. 

I'm tired now.