Its all because of the arrangement of elements in the periodic table...As we move to the right..The no of protons increases... The number of protons must be equal to that of electrons...electrons arrange themselves in the outer shells in the order 2,8,8,16......an so on......Thus...there usually the outer shell isn't complete so the number of electrons in the outer shell....valence -e.....suggests the number of electrons that can be donated or accepted... If you're below 15 it might be difficult for u ...a little....so u might check out the link and follow the lessons..it'll show ya how....
