The oxidation state, often called the oxidation number, is a theoretical charge assigned to an atom within a molecule or ion. This number represents the charge the atom would possess if all the electrons in its bonds were completely transferred to the more electronegative atom in each pair. It is a conceptual tool that helps chemists track the movement of electrons during chemical reactions, particularly in oxidation-reduction (redox) processes. An increase in an atom’s oxidation state corresponds to oxidation, or the loss of electrons, while a decrease signifies reduction, or the gain of electrons.
Establishing the Rules for Oxidation Numbers
Determining an element’s oxidation number in a compound requires following a hierarchy of universal rules. Any element in its uncombined, elemental form, such as molecular oxygen (O2) or solid zinc (Zn), is assigned an oxidation number of zero. For simple, single-atom ions, the oxidation number is equal to the charge of the ion, such as +1 for Na+ or -2 for S2-.
Alkali metals (Group 1) consistently exhibit a +1 oxidation number, and alkaline earth metals (Group 2) are always +2. Hydrogen is generally assigned a +1 oxidation number when bonded to nonmetals, though it takes a -1 state in metal hydrides, such as NaH.
Oxygen is typically assigned a -2 oxidation number in compounds, but this rule has exceptions, most notably in peroxides where it is -1, or when bonded to fluorine. The sum of all oxidation numbers in a neutral compound must equal zero. Conversely, the sum of the oxidation numbers in a polyatomic ion must equal the overall charge of that ion. This principle allows for the calculation of an unknown oxidation number for one element by using the known values of the others present in the structure.
Chlorine in Simple Ionic Compounds
Chlorine is a halogen, found in Group 17 of the periodic table, and it exhibits a wide range of oxidation states, from -1 to +7. In its natural, uncombined state, chlorine exists as a diatomic molecule (Cl2), where its oxidation number is zero, following the elemental rule. This zero state reflects that the two atoms share their electrons equally within the covalent bond.
The most common oxidation state for chlorine is -1, which occurs when the atom gains a single electron to achieve a stable, filled outer electron shell. This state is observed in all simple chloride salts and acids. For example, in sodium chloride (NaCl), sodium is +1, forcing chlorine to be -1 to maintain a neutral compound.
In hydrochloric acid (HCl), hydrogen is assigned its usual +1 state, requiring chlorine to be -1. This -1 state is its default and most stable condition, reflecting its strong tendency to act as an electron acceptor when paired with less electronegative elements.
How Chlorine Achieves Positive Oxidation States
Chlorine’s ability to achieve positive oxidation states, such as +1, +3, +5, and +7, is a direct result of its bonding with a more electronegative element, which is almost exclusively oxygen. When chlorine bonds with oxygen, the powerful electron-attracting nature of oxygen forces chlorine to formally “lose” electrons, resulting in a positive oxidation number. These positive states are most frequently seen in oxyanions, which are negatively charged ions containing both chlorine and oxygen atoms.
The hypochlorite ion (ClO-), famously found in household bleach, demonstrates the +1 oxidation state for chlorine. If the overall ion charge is -1 and oxygen is -2, the chlorine atom must be +1 for the values to sum correctly: (+1) + (-2) = -1.
Moving to the chlorite ion (ClO2-), the addition of a second oxygen atom raises the chlorine’s state to +3. Two oxygen atoms contribute a total of -4, meaning the chlorine must be +3 to achieve the -1 net charge: (+3) + 2(-2) = -1.
The chlorate ion (ClO3-) is a common example of chlorine exhibiting a +5 oxidation number. With three oxygen atoms contributing -6 to the total charge, the chlorine atom must be +5: (+5) + 3(-2) = -1.
Finally, the highest possible oxidation state for chlorine is +7, which is found in the perchlorate ion (ClO4-). This state occurs when chlorine is bonded to four oxygen atoms, where the +7 of the chlorine balances the total -8 from the four oxygen atoms to yield the -1 ion charge.