Core Concepts

Formal charge is an essential, simple concept to understand in stimulate to better understand molecule structures and reactions. In this tutorial, girlfriend will learn what is formal charge, exactly how to calculate it, and its significance in practice.

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Vocabulary

Ion: an atom or molecule v a network ionic charge, because of the existence or lack of electrons.Molecule: a group of atoms bonded togetherResonance: a term offered to explain the hybrid bonding in a molecule v multiple feasible structures.

What is official Charge?

Formal Charge is a fee assigned come an atom under the assumption that all electrons in binding are shared equally. This is a theoretical measure, no a genuine representation that the actual charge on an atom, which looks in ~ the ways electrons room actually shared between atoms in a bond. But much more on that later!

How come Calculate:

Formal charge (FC) = (# that valence electrons) – (½)(number of bonded electrons) – (number of unbonded electrons)

Examples:


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NH3: what is the formal charge on the nitrogen?

Nitrogen has actually 5 valence electrons, 6 bonded electrons (as there space 3 single bonds, every containing 2 electrons), and also 2 unbonded electrons in this configuration. Thus:

FC = (5) – (½)(6) – (2)FC = 0


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CH3O: what is the formal charge on the carbon?

Carbon has 4 valence electrons, 8 bonded electrons (two single bonds and also one double bond), and no unbonded electrons. Thus:

FC = (4) – (½)(8) – 0FC = 0


Note: though the formal fee in this two examples has been zero, that will not always be the case. Us will explore some examples of nonzero charge below.

Significance

1. Molecular Structure

Ideally, an atom in a molecule wants to have a formal charge of zero: this is the shortest energy, and thus the many stable state for it to be in. This hints us right into the structure of a molecule if there are multiple options: the one v the least/lowest official charges is the preferred structure. There room even certain guidelines to aid you number this out:

The preferred molecular framework is one wherein all officially charges room zero, as opposed come one whereby some officially charges are not zero.If there is no feasible structure where all official charges room zero, then the wanted structure is one through the least variety of nonzero charges.Adjacent atoms in a molecule should have opposite signs if charges space present.If there are multiple frameworks that meet requirements 1-3, then the structure with an adverse formal dues on the much more electronegative atoms is preferred.

Example: shown listed below are three possible structures because that N2O. Let’s number out which structure is correct.


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The height structure:First, we calculate the formal fee of the nitrogen on the left. Nitrogen has 5 valence electrons, this atom has actually 6 bonded electrons (a triple bond), and also 2 unbonded electrons, therefore the formal charge is (5) – (½)(6) – (2) = 0. Next, we calculate the formal fee of the nitrogen in the middle. This one has 8 bonded electrons and no unbonded, hence the formal fee is (5) – (½)(8) – (0) = +1. Finally, us calculate the formal charge of the oxygen. Oxygen has actually 6 valence electrons, and this atom has actually 2 bonded electrons and 6 unbonded, therefore the formal fee is (6) – (½)(2) – (6) = -1.The center structure:Similarly, we calculate the formal fee of the nitrogen ~ above the left: (5) – (½)(4) – (4) = -1.Next, the formal charge of the nitrogen in the middle: (5) – (½)(8) – (0) = +1.Finally, the formal fee of the oxygen: (6) – (½)(4) – (4) = 0. The bottom structure: Again, an initial we calculation the formal charge of the nitrogen ~ above the left: (5) – (½)(2) – (6) = -2.Next, the formal charge of the nitrogen in the middle: (5) – (½)(8) – (0) = +1. Finally, the formal fee of the oxygen: (6) – (½)(6) – (2) = +1.

Given this calculated officially charges, let’s consult the guidelines questioned above. First, space there any kind of structures feasible where all the formal charges are zero? There space not, so we move on to preeminence #2. This eliminates the bottom structure, as it has actually a greater number of nonzero charges than the peak two (it also has greater charges, as it includes a -2 charge, vice versa, the various other two just contain +/-1). Both the top and middle frameworks have surrounding atoms through opposite charges, therefore both satisfy dominance #3. This leaves dominance #4, definition the preferred structure is the one v the an unfavorable charge ~ above the much more electronegative atom. Oxygen is much more electronegative than nitrogen, an interpretation the wanted structure is the one v a an unfavorable charge top top the oxygen—the peak structure!

It is likewise worth noting that the sum of every the officially charges of the atoms in a molecule must equal the in its entirety charge on the molecule/ion. The is, they have to sum approximately zero if its an neutral molecule, and should sum up to the ion’s fee if that is not.

Example: the ion BH4– has an in its entirety charge of -1. This method that the formal charges of all the individual atom in the should add up come -1. Let’s watch if this is true.


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Boron has three valence electrons, eight bonded electrons, and also zero unbonded electrons. This provides its officially charge: (3) – (½)(8) – (0) = -1.

The four hydrogens in this molecule space all identical, therefore we can calculate all of their officially charges in ~ once. Hydrogen has actually one valence electron, 2 bonded electrons, and zero unbonded electrons. This renders its official charge: (1) – (½)(2) – (0) = 0.

As we have the right to see, the official charges include up come 0 + 0 + 0 + 0 + (-1) = -1. This amount does equal the overall charge ~ above the ion, which is -1.


2. Resonance

While formal charge deserve to indicate the desired structure of a molecule, as discussed above, the situation gets a bit more facility when there room multiple equally preferred structures. This situation may show resonance structures, specifically when the structures have actually the same arrangement of atoms, but different varieties of or arrangement of bonds.

Example: the diagram below shows three possible structures for the ion CO32-. We deserve to see that the setup of atoms is the same in all three structures (with the carbon in the center, connected to the three oxygens), but the placement of the double bond differs in every of the three.

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In each of them, the formal fee on the facility carbon is 0, the dual bonded oxygen is 0, and also the two solitary bonded oxygens space each -1. View if you deserve to calculate these you yourself correctly! note that as disputed above, 0 + 0 + (-1) + (-1) adds up to -2, i beg your pardon is the overall charge on the ion.

Since these bonds space the same in all three structures, their placement in the molecule is simply different, the official charges and also distributions of the in each structure are the exact same, definition that every of lock is equally likely to occur. This method that all three space correct structures, and in reality, the molecule develops a hybrid the all three structures.

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Read more about resonance frameworks here, and see much more examples here!

3. Reactivity

Finally, the formal fee can offer an indication as to how a molecule will certainly behave throughout a reaction. If an atom has a an adverse formal charge, the is more likely to it is in the resource of electrons in a reaction (a nucleophile). Whereas if it has a positive one, climate it is an ext likely to be expropriate electrons (an electrophile), and that atom specifically is most likely to be the website of the reaction.

Formal fee vs. Actual Charge

It is likewise important to not that formal charge is different from the actual charge of one atom. Formal charge does no take electronegativity into mind: it assumes that electrons in a bond are shared equally. It’s merely a formality, provided to aid make feeling of molecule structures and reaction mechanisms. Actual charge, top top the other hand, looks in ~ the really electron density, based upon the atoms’ electronegativities and also polarity the the bonds. To read more on these topics, inspect out these tutorials on: ion-dipole forces, periodic trends, and also polarity!