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Formal Charge vs Oxidation Number: Understanding the Key Differences

Formal Charge and

Oxidation Number: How they Differ? Formal charge and oxidation number are two concepts used to describe the distribution of electrons in chemical compounds.

These two concepts are related, but they have important differences that are often misunderstood. In this article, we will discuss formal charge and oxidation number separately, and then compare them to highlight the differences.

Formal Charge

Formal charge is a concept used to determine the most stable Lewis structure of a molecule or ion. The Lewis structure is a two-dimensional representation of a molecule or ion that shows the placement of all the atoms and bonds in the compound.

The formal charge of an atom in a molecule or ion is equal to the number of valence electrons minus the number of lone pair electrons minus one-half the number of bonding electrons. The formal charge is calculated for each atom, and the overall charge of the molecule or ion is determined by summing the formal charges of individual atoms.

To calculate the formal charge, the first step is to draw the Lewis structure of the molecule or ion. If the compound has resonance structures, each resonance structure has to be considered separately.

The valence electrons of each atom in the Lewis structure are counted. Valence electrons are the electrons involved in chemical bonding, and are represented by dots or lines in the Lewis structure.

The formal charge of an atom is then calculated as the difference between the number of valence electrons of the free atom and the number of valence electrons in the Lewis structure. Let us consider the carbon dioxide molecule (CO2) as an example.

The Lewis structure of CO2 has two double bonds between the carbon and oxygen atoms. The carbon atom has no lone pairs and four bonding electrons, while each oxygen atom has two lone pairs and two bonding electrons.

The formal charge of the carbon atom is therefore 4 – 0 – 1/2(8) = 0. The formal charge of each oxygen atom is 6 – 4 – 1/2(4) = 0.

The sum of the formal charges of the atoms in CO2 is zero. This indicates that the Lewis structure of CO2 is the most stable structure.

Another example we can consider is the nitrate ion (NO3-). The Lewis structure of NO3- has a single bond between the nitrogen atom and each oxygen atom, and a double bond between one of the oxygen atoms and the nitrogen atom.

Each oxygen atom has three lone pairs and one bonding electron, while the nitrogen atom has one lone pair and three bonding electrons. The formal charge of the nitrogen atom is therefore 5 – 2 – 1/2(6) = 0.

The formal charge of each oxygen atom is 6 – 4 – 1/2(2) = -1. The sum of the formal charges of the atoms in NO3- is -1.

This indicates that the ion has an overall negative charge of one. Formal charge is a useful concept in determining the most stable Lewis structure of a molecule or ion.

It is based on the distribution of electrons in a molecule or ion, and does not take into account any change in the oxidation state of the atoms.

Oxidation Number

Oxidation number is a measure of the number of electrons that an atom gains, loses or shares when it forms a chemical bond. Oxidation number is a formal charge assigned to an atom in a molecule or ion assuming that the electrons in the chemical bond are shared equally.

The oxidation number of an atom is determined by the number of valence electrons that the atom has, and the number of electrons that it gains, loses or shares when it forms a bond. For example, in ammonia (NH3), the nitrogen atom forms three single bonds with the hydrogen atoms.

The nitrogen atom has five valence electrons, and the hydrogen atoms each have one valence electron. The nitrogen atom gains one electron from each hydrogen atom when it forms the bond.

Therefore, the oxidation number of the nitrogen atom in NH3 is -3. The oxidation number of the hydrogen atoms is +1, as they lose one electron each when they form the bond.

In coordination complexes, the oxidation number is determined by assuming that all the ligands (atoms or molecules attached to the metal center) donate their electrons to the central metal atom equally. For example, in the complex ion [Co(NH3)6]3+, the oxidation number of the cobalt atom is +3.

This is because six ammonia molecules (each with one lone pair) donate six pairs of electrons to the cobalt atom, which has to be distributed evenly among all the ligands. Comparison with

Formal Charge

Both formal charge and oxidation number are measures of the distribution of electrons in molecules and ions.

However, there are important differences between these two concepts. Formal charge is a measure of the distribution of electrons in a specific Lewis structure of a molecule or ion.

It does not take into account the distribution of electrons in other possible resonance structures of the molecule or ion. In contrast, oxidation number is a measure of the distribution of electrons in all the bonds of a molecule or ion, assuming that the electrons in the bond are shared equally.

Another difference between formal charge and oxidation number is that formal charge does not consider changes in the oxidation state of the atoms. The oxidation state of an atom changes when there is a change in the number of electrons that the atom gains, loses or shares in a chemical reaction.

For example, in the reaction 2H2 + O2 2H2O, each hydrogen atom gains one electron, and each oxygen atom loses two electrons. Therefore, the oxidation state of hydrogen changes from 0 to +1, while the oxidation state of oxygen changes from 0 to -2.

In conclusion, formal charge and oxidation number are two important concepts in chemistry that are used to describe the distribution of electrons in molecules and ions. Although these two concepts are related, they have important differences that should be understood to avoid confusion.

Formal charge is a measure of the distribution of electrons in a specific Lewis structure of a molecule or ion, while oxidation number is a measure of the distribution of electrons in all the bonds of a molecule or ion, assuming that the electrons in the bond are shared equally. Formal charge does not consider changes in the oxidation state of the atoms, while oxidation number reflects changes in the oxidation state of the atoms.

In this article, we have explored the concepts of formal charge and oxidation number, which are essential measures to describe the electron distribution in chemical compounds. Formal charge determines the most stable Lewis structure of a molecule or ion, while oxidation number measures the electrons an atom gains, loses, or shares in a bond.

While these concepts are related, formal charge and oxidation number have important differences that should be understood to avoid confusion. Formal charge does not consider changes in the oxidation state of the atoms, while oxidation number reflects changes in the oxidation state of the atoms.

Understanding these measures allows researchers and students to accurately analyze and predict chemical reactions.

FAQs

1. What is formal charge?

Formal charge is a measure of the electron distribution in a molecule or ion that helps determine the most stable Lewis structure. 2.

How do you calculate formal charge? Formal charge is calculated as the valence electrons minus the lone pair electrons minus half the bonding electrons.

3. What is oxidation number?

Oxidation number measures the distribution of electrons in all the bonds of a molecule or ion, assuming that the electrons in the bond are shared equally. 4.

How do you determine oxidation number? Oxidation number is determined by the number of valence electrons that the atom has and the number of electrons it gains, loses, or shares when forming a bond.

5. What is the difference between formal charge and oxidation number?

Formal charge does not consider changes in the oxidation state of the atoms, while oxidation number reflects changes in the oxidation state of the atoms. Additionally, formal charge is calculated for a single Lewis structure, while oxidation number considers all the bonds in a molecule or ion.

6. Why are formal charge and oxidation number important in chemistry?

Formal charge and oxidation number help researchers and students to accurately analyze and predict chemical reactions by identifying the distribution of electrons in molecules and ions.

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