The valence electrons, the outermost shell of electrons, are the largest determinant of how an atom reacts chemically to other substances in its environment. The number and arrangement of the outermost electron shell are important, as is the placement of the shell itself, since different shells hold different numbers of electrons.
Chemical processes are involved in nearly all natural phenomena, including all the activities of life. The number of valence electrons is the biggest determinant of how any particular particle behaves chemically. Most valence shells hold up to eight electrons, while the outermost shells of hydrogen and helium only have room for two. The difference between the number of electrons in a neutral atom and the number of electrons needed to fill the outermost valence shell determines the reactivity of a given element.
When an atom or ion has only full electron shells, it is less chemically reactive; that is, it is less prone to form covalent bonds. The noble gases, such as helium and neon, are almost totally nonreactive because of their naturally full valence shells. Conversely, atoms only one or two electrons away from full valence shells, whether in the positive or negative direction, are the most reactive elements.