Elements of the Periodic Table are organized according to the Periodic Law. When  elements are ordered by increasing atomic number, one observes a repeating pattern or trend in how the elements' properties change. Properties such as atomic radius, ionization energy, and electronegativity change in a periodic and repeated manner. The elements are organized to reflect these trends. And because they are, we can make predictions about how the size of various elements compare to one another based on their location in the periodic table. 
 

What are Valence Electrons?
The valence electrons are the outer shell s and p electrons of an atom. There can be up to eight valence electrons in a single atom. Valence electrons are those electrons which are available for bond formation in covalently-bonded molecules. The total number of valence electrons in a molecule is simply the sum of the valence electrons contributed by all atoms in that molecule.


How does one Determine the # of Valence Electrons?
First you need to be able to determine the number of valence electrons present in each element of the compound. Since the number of valence electrons is a periodic property, the value can be easily determined by locating the element in the periodic table. As shown in the graphic below, all Group 1 elements have an s¹ electron configuration and thus have one valence electron. All Group 2 elements have an s² electron configuration and two valenece electrons. Group 13 elements have an s²p¹ electron configuration and three valenece electrons.  Similar logic can be applied to all other groups to obtain the results shown in the graphic below.

Once you have determined the number of valence electrons for the atoms of each element in the compound, you are ready to determine the total number of valence electrons in the molecule. To do so, add up all the valence electrons for each atom in the molecule of that compound. For instance, if you are trying to find the total number for valence electrons in CF₄, then you will need to first find out that carbon (C) has four valence electrons and fluorine (F) has seven valence electrons. Then add 4 + 7 + 7 + 7 + 7 to get a total of 32 valence electrons. The "7" is added four times since there are four fluorine atoms with 7 valence electrons each.