At the other end of the second-shell elements, neon is stable and unreactive because it already has a filled outer shell of eight electrons. Fluorine, with seven outer electrons, can achieve the neon configuration by picking up an electron from another atom to become a negative ion:


fluorine atomfluorine ion

The filled-shell structure of a fluoride ion is shown on the bottom right of the previous page. Carbon, in the center of the list, has four of the eight electrons needed to complete the second shell. One could imagine that it acquires four more electrons to become a quadruply charged negative ion:




with the neon arrangement, or alternatively that it loses four electrons to acquire the helium arrangement:




Neither process takes place in chemical reactions. Too much energy is required to pile up four positive or four negative charges on a tiny carbon atom.