Oxidation States

Introduction to Oxidation States

• The oxidation state of an element in a compound represents the hypothetical charge that an atom would have if all shared electrons were assigned to the atom with a greater attraction for electrons (more electronegative atom).
• An atom in a compound can be moreso oxidised (loses electrons), reduced (gains electrons), or remain in the neutral state.
• It serves to keep track of electrons in chemical reactions, particularly redox reactions where electron transfer is the main concept.

Assigning Oxidation States

• The oxidation state of an uncombined element is zero. For example, the oxidation state of Cl2, O2 or P4 is zero.
• The oxidation state of a monatomic ion is equal to the charge of that ion. For instance, Na^+ has an oxidation state of +1 and Cl^- has an oxidation state of -1.
• In compounds, the oxidation state of Fluorine is always -1 because it’s the most electronegative element.
• Oxygen usually has an oxidation state of -2 in compounds, though there are exceptions like in peroxides where it’s -1.
• Hydrogen has an oxidation state of +1 when combined with nonmetals and -1 when combined with metals.
• The sum of oxidation states of all elements in a compound equals zero, while in an ion, it equals the charge of the ion.

Oxidation and Reduction

• Oxidation refers to an increase in the oxidation state of an atom due to the loss of electrons.
• Reduction refers to a decrease in the oxidation state of an atom due to the gain of electrons.
• An atom that loses electrons in a chemical reaction is said to have been oxidised, while one that gains electrons has been reduced.

Common Misconceptions

• Remember that oxidation state does not always correspond to actual charge on an atom in a molecule or ion, it’s a hypothetical concept.
• It is vital to remember that elements such as fluorine, oxygen, and hydrogen have fixed oxidation states in compounds with some exceptions.

Importance

• Oxidation states are key in understanding many of the major concepts in chemistry like balancing redox reactions.
• They also have practical importance in the extraction of elements, particularly from seawater constituents in halogen production.
• Understanding of oxidation states has even more implications in environmental chemistry and corrosion studies.

Understanding the changes in oxidation states is the central theme in redox chemistry. This can be particularly challenging but will become more familiar with practice.