Using the Periodic Table you can easily find the number of valence electrons for an element. These are the electrons in the highest energy level and the ones that are involved in chemical bonding. This is an essential skill in chemistry!
In order to correctly draw Lewis Structures you must be able to find the number of valence electrons for a molecule. Essentially you are just adding up all the valence electrons for each element. But there are a few things to watch out for...