Oxidation Numbers of Atoms

In making drawings of atoms or molecules, the van der Waals radii may be used in indicating the volume within which the electrons are largely contained. For ions, the ionic radii (crystal radii) discussed in Section 6-10 may be used. The van der Waals radius of an atom and the ionic radius of its negative ion are essentially the same. For example, the van der Waals radius of chlorine is 180 pm, and the ionic radius of the chloride ion is 181 pm. 

The nomenclature of inorganic chemistry is based upon the assignment of numbers (positive or negative) to the atoms of the elements. These numbers, called oxidation numbers, are defined in the following way. 

The oxidation number of an atom is a number that represents the electric charge that the atom would have if the electrons in a compound were assigned to the atoms in a certain conventional way. 

The assignment of electrons is somewhat arbitrary, but the conventional procedure, described below, is useful because it permits a simple statement to be made about the valences of the elements in a compound without considering its electronic structure in detail and because it can be made the basis of a simple method of balancing equations for oxidation-reduction reactions. 

The oxidation number of a monatomic ion in an ionic substance is equal to its electric charge. 

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The oxidation number of atoms in an elementary substance is zero. 

Today, many reactions in aqueous solutions can be described as oxidation-reduction reactions (redox reactions). Oxidation is the process in which the oxidation number of atoms increases. Reduction is the process in which the oxidation number of atoms is decreased or made more negative. In another definition, oxidation is the loss of electrons by an atom, and reduction is the gain of electrons. Let us look at the following reaction ... 

Oxidation numbers of atoms / V / in a compound are the charges the atoms would have if the compound were ionic. 

Redox is shorthand for reduction and oxidation. Reduction is the gain of an electron by a molecule, atom, or ion. Oxidation is the loss of an electron by a molecule, atom, or ion. These two processes always occur together. Electrons lost by one substance are gained by another. In a redox process, the oxidation numbers of atoms are altered. Reduction decreases the oxidation number of an atom. Oxidation increases the oxidation number. 

In Section 16.1, we learned how to determine oxidation numbers of atoms of elements from the formulas of their ions or molecules. This section shows the opposite—how to write formulas for compounds based on knowledge of the possible oxidation numbers of the atoms of the elements. Predicting possible oxidation numbers is straightforward, but learning which are the most important oxidation numbers of even some of the most familiar elements takes a good deal of experience. 

Comparing oxidation numbers, you see that the zinc atom changes from 0 to +2 and that two hydrogen atoms change from +1 to 0. So, this is a redox reaction. In a redox reaction, the oxidation numbers of atoms that are oxidized increase, and those of atoms that are reduced decrease. 

A parameter that tracks the oxidation numbers of atoms in target bond forming reactions over the course of a synthesis plan relative to their values in the target molecule. 

FIGURE 5.3 Flowchart for determining oxidation numbers of atoms in chemical structures. 

In order to begin applying these steps, one needs to know how to determine the oxidation numbers of atoms in chemical structures in a systematic way. Figure 5.3 shows a flowchart that can be used as a guide. Equation 5.2 can be used to numerically evaluate the oxidation number of a given atom ... 

Oxidation-reduction reactions are reactions involving a transfer of electrons from one species to another or a change in the oxidation number of atoms. The concept of oxidation numbers helps us describe this t)q)e of reaction. The atom that increases in oxidation number is said to undergo oxidation the atom that decreases in oxidation number is said to undergo reduction. Oxidation and reduction must occur together in a reaction. Many oxidation—reduction reactions fall into the following categories combination reactions, decomposition reactions, displacement reactions, and combustion reactions. Oxidation—reduction reactions can be balanced by the halfreaction method. 

Note that Rule 5 requires that the sum of the oxidation numbers of atoms in the formula unit be equal to the charge on the unit, which is 0 for Mn02- There are two oxygen atoms, each at -2. The total contribution of oxygen is 2(-2), or -4. The sum of 4 plus the oxidation number of manganese is equal to 0. Manganese must therefore be +4. 

Oxidation-reduction reactions are identified by examining the changes in the oxidation numbers of atoms in the reactants and products. 

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