You may assume that the valences of the elements—the number of electrons with 
which an atom will bond or form—are those that can be derived by looking at the
groups (columns) of the periodic table. 
While these are the most common valences, the real behavior of electrons is less
simple.

Here is a table of element valences. Remember that an element's electron cloud 
will become more stable by filling, emptying, or half-filling the shell. 
Also, shells don't stack neatly one on top of another, so don't always assume an
element's valence is determined by the number of electrons in its outer shell. 

Table of Element Valences