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Formal Charge

When two or more Lewis structures with the same arrangement of atoms and number of electrons can be written, these are called resonance structures/forms. In reality, the actual structure is an average of the different possible Lewis structures.

Frequently Asked Questions about Formal Charge

The formula for formal charge is:

FC=number of valence electrons-number of lone pair electrons-number of bonds

Formal charge (FC) is the charge assigned to an atom is a molecule when we assume that electrons in all bonds are shared equally between atoms.

Using the Lewis structure, we can determine the number of bonds and lone pair electrons. Subtracting that from the number of valence electrons, we get the formal charge.

We use the formula for formal charge to calculate the formal charge for each individual atom. We then write that charge beneath each atom. The net charge is written on the top right of the molecule.

Formal charges tell us which Lewis structure is the ideal structure. Whichever Lewis structure has its formal charges closest  to zero is the correct structure.

Final Formal Charge Quiz

Question

Why do electrons in bonds get divided by two when counting towards formal charge?

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Answer

Electrons in bonds are shared, so it's assumed on average each atom only has one at a time

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Question

If hydrogen has 1 bond, what is its formal charge?

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Answer

0, hydrogen almost always only forms one bond and thus almost always has a formal charge of 0 in a molecule

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Question

How many resonance forms does CO2 have?

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Answer

3

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Question

How do you find the number of electrons in a valence shell of an atom?

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Answer

Looking at a periodic table to find the element and seeing what group it's in

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Question

If a carbon atom has 2 bonds and 2 electrons in a molecule, what is its formal charge?

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Answer

+1

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Question

Are molecules with the same structure but with a different amount of electrons resonance structures?

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Answer

No, a different number of electrons means it's a totally different molecule

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Question

Why are formal charges typically assigned after looking at a molecule in a Lewis structure?

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Answer

Using a Lewis structure allows us to find the number of bonds and lone pairs an atom will have

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Question

True or false: If two or more oxygen atoms are in a molecule, they must have the same formal charge

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Answer

False! No two or more of the same element must have the same formal charge in a molecule

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Question

True or false: Formal charge can be used to determine the most commonly occurring form of a molecule?

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Answer

True! This is why assigning formal charge in Lewis structures is so important

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Question

If a chlorine atom has 4 bonds and 2 lone pairs, what is its formal charge?

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Answer

+1

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Question

What is formal charge?

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Answer

Formal charge (FC) is the charge assigned to an atom is a molecule when we assume that electrons in all bonds are shared equally between atoms. 

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Question

Which of the following is NOT true about formal charge?

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Answer

If there are multiples of the same element, they will have the same formal charge

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Question

What are valence electrons?

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Answer

Valence electrons are the electrons that exist in the highest energy level/shell. They are the electrons that participate in bonding.

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Question

Which of the following are reasons why we assign a formal charge? Select all that apply

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Answer

To find the best Lewis structure

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Question

What are lone pairs?

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Answer

Lone pairs are a set of valence electrons that do not participate in bonding. Because of this, they are also called non-bonding electrons. 

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Question

What is the formula for formal charge?

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Answer

$$FC=(\text{number of valence electrons})-(\text{number of lone pair electrons})-(\text{number of bonds})$$

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Question

How do we use the Lewis structure(s) to determine formal charge?

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Answer

Lewis structures tell us the number of bonds and lone pair electrons

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Question

What are resonance structures?

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Answer

When two or more Lewis structures with the same arrangement of atoms and number of electrons can be written, these are called the resonance structures/forms. 

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Question

True or False: For resonance structures, the "true" structure is an average of all the resonance forms.

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Answer

True

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Question

True or False: Formal charges must add up to 0

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Answer

False

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