Formal Charge - Online Tutor, Practice Problems & Exam Prep

Formal charge is a concept used to assign a charge to an atom in a molecule, assuming that electrons are shared equally among atoms, regardless of differences in electronegativity. It is calculated using the formula:

$\mathrm{formal}\mathrm{charge}=\mathrm{valence}\mathrm{electrons}-\mathrm{number}\mathrm{of}\mathrm{bonds}-\mathrm{nonbonding}\mathrm{electrons}$

Valence electrons correspond to the group number of the element in the periodic table, while nonbonding electrons are counted individually. Bonding electrons are involved in chemical bonds, whereas nonbonding electrons do not participate in bonding. By determining the formal charge of each atom in a molecule, we can compute the net charge of the compound, which is the sum of all formal charges. Understanding formal charge is crucial in predicting the stability and reactivity of molecules in chemical reactions.

Formal Charge

Now, formal charge represents the charge given to elements when assuming electrons are shared equally regardless of electronegativity. In reality, sometimes an element that's more electronegative will hold on to that electron within the electrons within the bond a little bit more tightly around themselves, which inflates or makes their electron clouds larger. But when it comes to formal charge, we don't take that into consideration. We assume that all atoms that are sharing electrons between each other, they share them equally.

Now with formal charge, we're introduced to terms such as bonding electrons versus non bonding electrons. Bonding electrons are the electrons that do participate in bonding with other elements and nonbonding electrons are the electrons that don't participate and bonding with other elements. With formal charge we have our formal charge formula here. We'd say formal charge equals the valence electrons of that element that we're examining minus the bonds that it is making plus the non bonding electrons that it possesses.

We're going to say remember valence electrons equals the group number of that element. And when we're talking about non bonding electrons, realize that we're going to count them individually with formal charge. When we find the formal charge of every element within a compound, we can calculate what's called the net charge. The net charge is the overall charge. This is the sum of all formal charges within a compound.

So now that we know the formal charge formula, click on to the next video and let's put it to practice with the given example.

Formal Charge Example 1

Here we must determine the formal charge of the nitrogen atom found within the ammonia molecule, which is NH₃. All right, So we need formal charge, which is just going to abbreviate it. FC equals valence electrons, which is equal to the group number of the element.

Nitrogen is in Group 5A, so it's 5 minus the bonds that it's making. It is making 123 bonds plus its non bonding electrons, the electrons that are not participating in bonding, so they're counted individually. This would be 1/2 nonbonding electrons, so this would be

5 - ( 3 + 2 )

, so it would be

5 - 5

.

So we'd have a formal charge of 0 for the nitrogen atom within ammonia. This means that option C would be our correct answer.

Calculate the formal charges for each of the oxygen atoms within the nitrite ion, NO₂^–.

• A. 0, -1
• B. +1, 0
• C. -1, -1
• D. +1, +1

Calculate the formal charge of the carbon atom within a carbon monoxide molecule.

• A. –1
• B. +2
• C. 0
• D. +1
• E. –2

Based on calculated formal charges, determine the overall charge (net charge) for the following compound.

• A. –3
• B. +4
• C. +1
• D. –1
• E. +2

Which element within the thiocyanate ion possesses a negative charge?

• A. C
• B. S
• C. N
• D. All of them.
• E. None of them.

Here’s what students ask on this topic:

How do you calculate formal charge?

To calculate the formal charge on an atom in a molecule or polyatomic ion, you can use the following formula:

$\text{Formal Charge}=\left(\text{Valence Electrons}\right)-\left(\text{Non-bonding Electrons}\right)-(\text{Bonding Electrons}/2)$

Here's how to apply the formula step by step:

1. Valence Electrons: Determine the number of valence electrons for the atom in its ground state (as if it were not bonded to any other atoms). You can find this information on the periodic table based on the element's group number.
2. Non-bonding Electrons: Count the number of non-bonding electrons on the atom. These are the electrons that are not involved in bonding and are typically represented as dots in Lewis structures.
3. Bonding Electrons: Count the number of electrons that the atom shares with other atoms in bonds. Each single bond counts as two electrons, a double bond as four, and so on.
4. Divide Bonding Electrons by 2: Since bonding electrons are shared with another atom, you divide the number by 2 to get the number of electrons that are "owned" by the atom in question.

Subtract the sum of the non-bonding electrons and half the bonding electrons from the number of valence electrons to get the formal charge. The result

What is formal charge?

Formal charge is a concept used in chemistry to help predict the distribution of electrons among atoms within a molecule. It's a way to quantify how much an atom in a molecule deviates from its valence electrons in its elemental state. To calculate the formal charge of an atom, you use the formula:

$\text{Formal Charge}=\left(\text{Valence Electrons}\right)-\left(\text{Non-bonding Electrons}\right)-(\text{Bonding Electrons}/2)$

Here, valence electrons are the number of electrons an atom has in its outer shell when it's not bonded to other atoms. Non-bonding electrons are the electrons in lone pairs that are not shared with other atoms. Bonding electrons are those shared in bonds with other atoms, and since these electrons are shared, you divide by two to allocate them equally to the two bonded atoms.

The sum of the formal charges in a neutral molecule must be zero, while in an ion, it should equal the ion's charge. Formal charges help in drawing Lewis structures, predicting molecular geometry, and understanding reaction mechanisms.

What does formal charge tell you?

Formal charge is a concept used in chemistry to help understand the distribution of electrons in a molecule. It is calculated for each atom by taking the number of valence electrons in the free, neutral atom, subtracting the number of non-bonding (lone pair) electrons, and then subtracting half the number of bonding (shared) electrons. The formal charge is used to determine the most likely structure of a molecule by comparing the possible structures, known as resonance structures. Structures with formal charges closest to zero are generally more stable.

The formal charge can indicate the relative electronegativity of atoms in a molecule, suggesting which atoms are more likely to attract electrons. It's also helpful in assessing the reactivity of molecules, as atoms with a positive formal charge are typically electrophilic (electron-seeking) and those with a negative formal charge are nucleophilic (nucleus, or positive charge, seeking). However, it's important to note that formal charge is a theoretical charge and does not necessarily represent the actual charge on an atom within a molecule.