Determine the number of valence electrons for the element using the periodic table. The periodic table is arranged such that the columns correspond to an increasing number of valence electrons from left to right (excluding the transition metals). Thus, columns 1A through 7A have a corresponding number of valence electrons for all of the elements in the respective columns (i.e. column 1A elements all have 1 valence electron and column 2A elements all have 2 valence electrons). Using oxygen as an example, we see that oxygen (O) is located in column 6A and thus has 6 valence electrons.
Draw the electrons as dots, one by one, around the element. We need to draw 6 dots (valence electrons) around the symbol of oxygen, O. Visualize the O with the potential to have 2 electrons on each of its four sides for a total of 8 valence electrons (Octet Rule). Start by drawing the first dot on any side of the element and continue adding one electron to each side in a clockwise or counterclockwise fashion ensuring that each side has at least one electron before adding a second to any of the sides. There should be six total valence electrons for oxygen upon completion (2 sides with 2 electrons and 2 sides with 1 electron).
Determine the significance of the diagram for the element. We see in the oxygen diagram that it is missing two valence electrons from its outer shell for each side that only has one. This means that oxygen needs two electrons from some other element to complete its octet. This could occur through a single element with two single electrons in its valence shell, such as another oxygen element (O2 is the formula or oxygen gas), or from two separate elements that each have one valence electron to share, as seen with two hydrogen molecules each having one valence electron combining with a single oxygen molecule to form water.