An oxidation state refers to the hypothetical electrical charge an atom would have if its bonds were completely ionic. This value indicates how many electrons an atom has effectively gained or lost when forming a compound. While many elements, such as sodium or calcium, consistently exhibit only one stable charge, a significant number of elements possess the ability to exhibit two or more different oxidation states, known as variable charge. This flexibility allows the same element to form entirely different compounds, making accurate identification of the specific charge fundamental to chemistry.
The Electron Configuration Behind Variable Charges
The ability of an element to have multiple charges stems directly from the arrangement of its electrons, specifically the involvement of both the valence and sub-valence electrons. Elements with variable charges, particularly the transition metals, have electrons in the inner d-orbitals that are relatively close in energy to the outermost s-orbital electrons.
When the atom forms an ion, it typically loses the electrons from the highest-energy s-orbital first, often resulting in a +2 charge. Because the inner d-orbital electrons are energetically accessible, the atom can sometimes lose one or more of these d-electrons as well. Losing a different number of these inner d-electrons leads to a second stable oxidation state, such as a +3 or +4 charge.
Identifying the Main Groups of Elements with Multiple Charges
The majority of elements displaying multiple stable charges are the Transition Metals, found in the large central block of the periodic table (Groups 3 through 12). This group is characterized by the sequential filling of the d-orbitals, which accounts for their variable oxidation states. Iron commonly forms two different ions: iron(II) (+2) and iron(III) (+3). Copper can exist as copper(I) (+1) or copper(II) (+2).
P-Block Elements
Certain elements in the p-block (Groups 13–16) also have variable charges. For example, the heavier metals in Group 14, such as tin and lead, can form ions with both a +2 and a +4 charge. This variation is often attributed to the “inert pair effect,” where the two outermost s-electrons are sometimes reluctant to participate in bonding.
Lanthanides and Actinides
The Lanthanides and Actinides, located at the bottom of the table, also exhibit complex and variable charges. Their specialized chemistry is due to the involvement of the inner f-orbitals.
Communicating Variable Charges in Chemical Nomenclature
Because an element like iron can form multiple different compounds depending on its charge, chemists need a clear system to distinguish between them. The modern and internationally accepted method is the IUPAC Stock System, which uses Roman numerals to specify the exact charge of the metal ion. When naming an ionic compound, the Roman numeral is placed in parentheses immediately after the name of the metal. For example, \(\text{FeO}\) (where iron has a +2 charge) is named iron(II) oxide, while \(\text{Fe}_2\text{O}_3\) (where iron has a +3 charge) is named iron(III) oxide. Similarly, the two forms of copper chloride are systematically named copper(I) chloride (\(\text{CuCl}\)) and copper(II) chloride (\(\text{CuCl}_2\)).
Older Naming Conventions
An older, less common naming convention uses the suffixes -ous and -ic appended to the Latin root of the element’s name. In this older system, the suffix -ous indicated the lower charge state, and -ic indicated the higher charge state. For example, \(\text{Fe}^{2+}\) was called ferrous and \(\text{Fe}^{3+}\) was called ferric.