Nonmetals increase in chemical activity as you go from left to right on a horizontal row and from bottom to top in a vertical column. The elements in the far right-hand column on the periodic table are chemically inert. Most of the families on the periodic table are in vertical columns. An element's oxidation number, sometimes called valence, is the number of electrons gained or lost by an atom when forming compounds. This characteristic is controlled by the electrons in the outer energy level.
Atoms gain or lose electrons to get eight electrons in their outer shell .
The shape of the periodic table makes the four sublevels easy to see. Helium is at the far right because it is inert. For the purpose of reading sublevels, it should be thought of as sitting in the second column beside hydrogen. One thing that does not show on the periodic table is the overlap of some sublevels. Both the d and f sublevels overlap the s sublevel of the next lowest electron energy level.