A covalent bond between two atoms results when those atoms share electrons with one another. The electrons that are being shared are valence electrons (or outer shell electrons). To determine how electrons are shared among atoms in a molecule, its important to first be able to count the number of valence electrons within each atom
 

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.