For purposes of describing chemical behavior, an atom can be considered as a positively charged nucleus surrounded by negatively charged electrons orbiting in concentric spherical shells. The number of positive charges in the nucleus determines how many electrons normally surround the nucleus; as atomic number increases, the electron shells are filled, starting with those nearest the nucleus.
The valence of an atom is determined by the number of electrons in the outermost, or valence, shell. The atom exists in its most stable configuration when its outermost shell is completely filled; in combining with other atoms, it thus tends to gain or lose valence electrons in order to attain a stable configuration. If the valence shell of the atom is nearly complete, as in chlorine and other nonmetals, the atom will tend to accept electrons to complete it; if the valence shell has few electrons, as in potassium and other metals, the atom will tend to lose these electrons, so that the next shell below the valence shell becomes a completed outermost shell.
The valence of many elements is determined from their ability to combine with hydrogen or to replace it in compounds. For example, one oxygen atom combines with two hydrogen atoms to form water and the valence of oxygen is thus determined to be 2. Similarly, chlorine accepts one electron in combining with a single atom of hydrogen to form hydrogen chloride, HCl, and chlorine's valence is 1. Zinc does not combine with hydrogen but does replace it in compounds; in a typical replacement reaction, one zinc atom replaces two hydrogen atoms, as in the equation Zn+H2SO4→ZnSO4+H2, so that zinc has a valence of 2.
Atoms are assigned numbers, called valence numbers, oxidation numbers, or oxidation states, which range in value from -4 through 0 to +7 and describe the combining behavior of the atoms in chemical reactions, particularly oxidation-reduction reactions (see oxidation and reduction). Metals, which commonly donate electrons and form compounds in which they exist in the positive, or cationic, state, are assigned positive oxidation numbers (see cation). For a metal such as zinc, which donates two electrons to achieve a stable electron configuration, the oxidation number is +2. Nonmetals, which commonly accept electrons and in compounds exist in the negative, or anionic, state, are assigned negative oxidation numbers (see anion). The oxidation number is -1 for chlorine and the other halogens, which accept one electron to complete their valence shell.
Some elements, like the transition metals, have electron configurations in which electrons from their inner shells can also be used as valence electrons; these elements can have several different oxidation states. For example, iron can have a valence of +2 or +3, and chromium can have a valence of +2, +3, or +6. Iron in the +3 oxidation state, Fe+3, acts as an oxidizing agent, accepting one electron to attain the Fe+2 state, while ferrous iron, Fe+2, by donating an electron in going to the +3 state, acts as a reducing agent.