Ok buddy- so if you grab your periodic table there should be numbers on the side... (REFER TOO IMAGE ONE) So then we find them on the periodic table... (REFER TO IMAGE TWO) and if we look at the number the higher it gets the more radioactive it gets, and thats why it stops because the atomic gets so high that the level can't even be able to be mad because it will just explode and when it does that its dangerous because of how radioactive it is... so if we look the more "gas" concentrated elements are seen at the top (i.e. the helium) So the order would be He, C, O, Ne, Ca, I
Then just do that for the rest of them and the numbers on the side are the positive (+) or negative (-) amount of atoms that the atom is missing or ha to be stabilized or over stabilized, but you will learn that later on when you go into periodic equations.
Hopefully this helped, have a wonderful day! :)