Oxidation state

Oxidation state, also referred to as the oxidation number, defines the level of oxidation present in an atom as determined by the addition of oxygen atoms, the loss of electrons, or the loss of hydrogen atoms. The oxidation state signifies the electrical charge of an atom when it forms a chemical bond. The charges of each atom in a bond should add up to zero when combined, but this is not always the case. Although the terms oxidation state and oxidation number are used interchangeably, the two terms are slightly different. Oxidation state describes the degree of oxidation experienced by an atom in a molecule if all bonds formed were ionic bonds, or bonds in which one or more electrons are transferred between atoms. The total of each atom’s oxidation states should equal the total charge of the molecule or ion. Oxidation states are determined by a set of rules based on information gleaned from the periodic table and an atom’s electronegativity—the tendency of an atom to attract pairs of electrons. The term oxidation number when defined separately from oxidation state can also refer to the charge of a central atom of a bond by itself.

Background

Oxidation has three distinct but linked definitions. Oxidation can refer to the addition of oxygen atoms to an element or compound and the resultant reactions that occur in those elements and compounds. Oxidation also refers to the loss of electrons from a chemical element or compound. Oxidation can also be defined as the loss of hydrogen atoms. These definitions are related to one another, since oxidation is caused by the loss of electrons and hydrogen atoms due to the addition of one or more oxygen atoms. The definitions are also distinct because the addition of oxygen atoms or the loss of hydrogen atoms is not always necessary for oxidation to occur. The opposite of oxidation reactions is reduction reactions, which involve the loss of oxygen atoms, the gaining of electrons, or the gaining of hydrogen atoms.

The term oxidation was first coined by French chemist Antoine Lavoisier. He used the term to define the reaction that occurred when a substance mixed with oxygen. From his research, scientists were able to determine the exact process of oxidation. When a substance is oxidized, it loses electrons, so the definition of oxidation was later broadened to include any reaction in which electrons are lost regardless of oxygen’s involvement.

In terms of oxidation by a loss of electrons, the electrons are lost by the reactant, or the substance undergoing the chemical reaction. Those lost electrons are gained by the product, the substance created by a chemical reaction. An example would be the reaction that occurs when a sodium metal combines with a chlorine gas. In order for the product to be balanced, the sodium metal (Na⁺) must lose an electron, which is gained by the chlorine gas (Cl^-). Combined, the sodium and chlorine become sodium chloride (NaCl), also known as table salt. Due to the loss of an electron, sodium is therefore said to have been oxidized, becoming a sodium cation in turn. A cation is a type of ion, or an atom that has acquired an electric charge, that has a positive charge due to the loss of electrons.

In terms of the addition of oxygen to a substance, oxidation occurs when one substance reacts with oxygen either in its elemental or gas form. For example, when oxygen gas (O₂) is added to carbon (C), the product is carbon dioxide (CO₂). This reaction occurs during the burning of coal and is regarded as a combustion reaction due to the use of heat. The rusting of iron is another example of oxidation by the gaining of oxygen atoms, with Iron (Fe) combining with oxygen gas (O₂) to produce iron oxide (Fe₂O₃), otherwise known as rust. In other reactions, oxidation can occur when a hydrogen atom is lost. When a substance containing hydrogen is broken down into its component parts, sometimes one or more hydrogen atoms are lost in the process. For example, when methyl alcohol (CH₃OH) is broken down, it becomes formaldehyde (CH₂O [l] + H₂ [g]). The original compound starts out with four hydrogen atoms but upon breaking down, the product only has two hydrogen atoms.

Overview

To keep track of just how chemical compounds and elements undergo oxidation, scientists began assigning what became known as oxidation numbers, or oxidation states, to atoms. This number refers to the total number of electrons an atom either gains or loses in forming a chemical bond with another element. Oxidation states apply to oxidation-reduction reactions. Each atom involved in these reactions is assigned an oxidation number that illustrates the atom’s ability to gain, lose, or share electrons. For example, the oxygen atom has an oxidation number of -2 (O^2-), which means it is capable of donating two electrons to another atom in a chemical reaction. The iron atom, on the other hand, can have an oxidation state of either +2 or +3 (Fe²⁺, Fe³⁺), meaning it can gain either two or three electrons to form a chemical bond with another atom. In molecules with a neutral charge, the sum of the oxidation numbers of each atom in a compound adds up to zero. However, some molecules and ions have charges greater than or less than zero, so the sum of each atom’s oxidation numbers would equal the total charge of the molecule or ion.

As illustrated above, some elements have only one oxidation state, while others can have multiple oxidation numbers. Fluorine has an oxidation number of -1 in all compounds. Other elements, specifically non-metals and transition metals, can have multiple oxidation numbers. Nitrogen and iron are two common examples of elements with more than one oxidation number. When an atom is capable of having more than one oxidation number, its particular oxidation state at the time of the reaction is indicated by placing roman numerals in parentheses next to the atomic symbol, for example, Fe³⁺ is referred to as iron (III).

Bibliography

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