An atom can have the following charges: positivenegativeor neutraldepending on the electron distribution. This is often useful for understanding or predicting reactivity.

**Formal Charges: Calculating Formal Charge**

Identifying formal charges helps you keep track of the electrons. The formal charge is the charge on the atom in the molecule. It is actually spread out through the other atoms and is not only on the one atom. Identifying a formal charge involves:.

Lewis structures also show how atoms in the molecule are bonded. They can be drawn as lines bonds or dots electrons. One line corresponds to two electrons. The nonbonding electrons, on the other hand, are the unshared electrons and these are shown as dots. One dot is equal to one nonbonding electron. The valence electrons are the electrons in the outermost shell of the atom. A number of bonding electrons: 2 for H, 8 for C.

These hydrogens are all zero. A number of bonding electrons: 2 for H, 6 for C. If you have any questions or would like to share your reviews on the How to calculate formal charge, then comment down below.

I would love to hear what you have to think. Save my name, email, and website in this browser for the next time I comment. Table of Contents How to calculate formal charge Examples. How to calculate formal charge ot all atoms within a neutral molecule need be neutral. Determining the appropriate number of valence electrons for an atom โ This can be accomplished by inspecting the periodic table. The group number indicates the appropriate number of valence electrons for each atom Determining whether the atom exhibits the appropriate number of electrons โ In the Lewis structure, determine whether some of the atoms show an unexpected number of electrons.

The formal charge on an atom can be calculated using the following mathematical equation. Leave a Reply Cancel reply. Close Menu. Are you ready for your next Ochem Exam?

Cheat Sheets.In chemistrya formal charge FC is the charge assigned to an atom in a moleculeassuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity. In contrast, this convention is not followed in inorganic chemistry. Formal charges are drawn in close proximity to the atom bearing the charge. They may or may not be enclosed in a circle for clarity. On the other hand, many workers in organometallic and a majority of workers in coordination chemistry will omit formal charges, unless they are needed for emphasis, or they are needed to make a particular point.

This difference in practice stems from the relatively straightforward assignment of bond order, valence electron count, and hence, formal charge for compounds only containing main-group elements though oligomeric compounds like organolithium reagents and enolates tend to be depicted in an oversimplified and idealized mannerwhile there are genuine uncertainties, ambiguities, and outright disagreements when these assignments are attempted for transition-metal complexes.

Even though all three structures gave us a total charge of zero, the final structure is the superior one because there are no charges in the molecule at all. It is important to keep in mind that formal charges are just that โ formalin the sense that this system is a formalism.

The formal charge system is just a method to keep track of all of the valence electrons that each atom brings with it when the molecule is formed. Formal charge is a tool for estimating the distribution of electric charge within a molecule. If the formal charges and oxidation states of the atoms in carbon dioxide are compared, the following values are arrived at:. The reason for the difference between these values is that formal charges and oxidation states represent fundamentally different ways of looking at the distribution of electrons amongst the atoms in the molecule.

With formal charge, the electrons in each covalent bond are assumed to be split exactly evenly between the two atoms in the bond hence the dividing by two in the method described above. The formal charge view of the CO 2 molecule is essentially shown below:. The covalent sharing aspect of the bonding is overemphasized in the use of formal charges, since in reality there is a higher electron density around the oxygen atoms due to their higher electronegativity compared to the carbon atom.

This can be most effectively visualized in an electrostatic potential map. With the oxidation state formalism, the electrons in the bonds are "awarded" to the atom with the greater electronegativity. The oxidation state view of the CO 2 molecule is shown below:. Oxidation states overemphasize the ionic nature of the bonding; the difference in electronegativity between carbon and oxygen is insufficient to regard the bonds as being ionic in nature.

In reality, the distribution of electrons in the molecule lies somewhere between these two extremes. The inadequacy of the simple Lewis structure view of molecules led to the development of the more generally applicable and accurate valence bond theory of SlaterPaulinget al.Formal charge is the charge we assign to a bonded atom if the bonding electrons were shared equally between the bonded atoms. Sometimes we can write more than one Lewis structure for a particular ion or molecule.

When that happens, we usually assign formal charges to the bonded atoms to help determine the correct Lewis structure. After applying the rules outlined above to each atom in the Lewis structure, we will then use the following formula to calculate the formal charge of each atom:.

Once we add all the formal charges for the atoms in the Lewis structure, we should get a value equal to the actual charge of the molecule or ion. If it is a neutral molecule, then the sum of all the formal charges must equal zero. If it is a molecular ion, then the sum of all the formal charges must equal the ionic charge. After assigning formal charges, we again apply the following rules to identify the correct Lewis structure:.

To draw the Lewis structure for HCNwe will first calculate the total number of valence electrons. If you look on the periodic table, you will notice that H has one valence electron, C has 4and N has 5.

If you sum all these valence electrons, you will get Next, we identify the central atom. But we know that hydrogen can form only a single bond, this means H cannot be the central atom. Since H cannot be the central atom, it follows that only C or N can be the central atom.

If carbon is the central atom, then the bond skeleton of its Lewis structure will appear as:. If N is the central atom, then the bond skeleton of its Lewis structure will appear as:. To distribute the 6 electrons, we start from the outer atoms, but since H can have only 2 valence electrons and it does have its 2 valence electrons, we move on to N in the bond skeleton a.

As you can see, N needs only 6 more electrons to have an octet, so we give all the 6 electrons to it. Therefore, the Lewis structure of a will appear as:. However, notice C arbon has only 4 electrons, but needs 4 more to satisfy the octet rule. This means, we must move two lone pairs of electrons from N to form multiple bonds between C and N. If we do, the completed Lewis structure with carbon in center will appear as:.

To determine that, we must assign formal charges. To determine the formal charge of Hwe must first figure out how many electrons it owns in the Lewis structure. From the rules outlined above, we do know it owns half of the shared electrons between it and C.

And since there is a single bond between it and Cit follows that H owns only 1 electron 2 electrons divided by 2. Now, to determine the formal charge of H, we will simply subtract 1 from the valence electron of H predicted by the periodic table.

Therefore, the formal charge of H is zero. Now, if we look at Lewis structures e and f with formal charges, we can predict with reason that structure e should be stable. We think so because all the atoms in f have a formal charge of zero.

Since the negative charge should reside on the most electronegative atom, if follows that Lewis structure f is incorrect unstable. Why do we have to assign formal charges?

### Formal Charge

To determine the formal charge for an atom, we usually follow these rules: Assign all lone pairs of electrons to the atom on which we find them Assign half of the bonding electrons to each atom in the bond After applying the rules outlined above to each atom in the Lewis structure, we will then use the following formula to calculate the formal charge of each atom: How to calculate formal charge Once we add all the formal charges for the atoms in the Lewis structure, we should get a value equal to the actual charge of the molecule or ion.

Bond skeleton with nitrogen in center. Incomplete Lewis structure. Complete Lewis structure. Lewis structure with formal charges.

I teach chemistry at Wright State University.Resonance structures are all the possible Lewis structures for a molecule. Formal charge is a technique to identify which resonance structure is the more correct structure. The most correct Lewis structure will be the structure where the formal charges are evenly distributed throughout the molecule.

The sum of all the formal charges should equal the total charge of the molecule. Formal charge is the difference between the number of valence electrons of each atom and the number of electrons the atom is associated with. The equation takes the form:. The two resonance structures in the above picture are for carbon dioxideCO 2. To determine which diagram is the correct one, the formal charges for each atom must be calculated.

To find e Ncount the number of electron dots around the atom. To find e Bcount the bonds to the atom.

## Formal charge

Each bond is formed by two electrons, one donated from each atom involved in the bond. Multiply each bond by two to get the total number of electrons. Use these three values to calculate the formal charge on each atom. All the formal charges on Structure A equal zero, where the formal charges on Structure B show one end is positively charged and the other is negatively charged.

Share Flipboard Email. Todd Helmenstine. Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. Updated February 04, It is more important that students learn to easily identify atoms that have formal charges of zero, than it is to actually calculate the formal charge of every atom in an organic compound.

Students will benefit by memorizing the "normal" number of bonds and non-bonding electrons around atoms whose formal charge is equal to zero. A formal charge compares the number of electrons around a "neutral atom" an atom not in a molecule versus the number of electrons around an atom in a molecule.

Formal charge is assigned to an atom in a molecule by assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity. To calculate formal charges, we assign electrons in the molecule to individual atoms according to these rules:. A neutral nitrogen atom has five valence electrons it is in group From the Lewis structure, the nitrogen atom in ammonia has one lone pair and three bonds with hydrogen atoms.

Substituting into Equation 2. A neutral hydrogen atom has one valence electron. Each hydrogen atom in the molecule has no non-bonding electrons and one bond. Using Equation 2.

The sum of the formal charges of each atom must be equal to the overall charge of the molecule or ion. In this example, the nitrogen and each hydrogen has a formal charge of zero.

When summed the overall charge is zero, which is consistent with the overall neutral charge of the NH 3 molecule. Typically, the structure with the most formal charges of zero on atoms is the more stable Lewis structure. In cases where there MUST be positive or negative formal charges on various atoms, the most stable structures generally have negative formal charges on the more electronegative atoms and positive formal charges on the less electronegative atoms.

The next example further demonstrates how to calculate formal charges for polyatomic ions. The nitrogen atom in ammonium has zero non-bonding electrons and 4 bonds. Each hydrogen atom in has one bond and zero non-bonding electrons.

The formal charge on each hydrogen atom is therefore. Adding together the formal charges on the atoms should give us the total charge on the molecule or ion. The calculation method reviewed above for determining formal charges on atoms is an essential starting point for a novice organic chemist, and works well when dealing with small structures. But this method becomes unreasonably time-consuming when dealing with larger structures. It would be exceptionally tedious to determine the formal charges on each atom in 2'-deoxycytidine one of the four nucleoside building blocks that make up DNA using equation 2.

And yet, organic chemists, and especially organic chemists dealing with biological molecules, are expected to draw the structure of large molecules such as this on a regular basis. Clearly, you need to develop the ability to quickly and efficiently draw large structures and determine formal charges. Fortunately, this only requires some practice with recognizing common bonding patterns.

Carbon, the most important element for organic chemists. In the structures of methane, methanol, ethane, ethene, and ethyne, there are four bonds to the carbon atom. And each carbon atom has a formal charge of zero.

In other words, carbon is tetravalentmeaning that it commonly forms four bonds. Carbon is tetravalent in most organic molecules, but there are exceptions. Carbocations occur when a carbon has only three bonds and no lone pairs of electrons. Carbanions occur when the carbon atom has three bonds plus one lone pair of electrons. Carbanions have 8 valence electrons and a formal charge of Two other possibilities are carbpon radicals and carbenes, both of which have a formal charge of zero.

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