Each element has an elemental structure for example, Flourine is the 9th element in the table so you would draw a circle with 2 circles on it representing electrons, then another circle around that because their is 7 electrons remaining. You only do two on the first ring because the order is 2, 8, 8. So, if you have the fifteenth element you do two, eight and then five because the third ring filled up.
This is what flourine would look like:
