Formal Charge: Videos & Practice Problems

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

Formal charge is a concept used to assign a charge to atoms in a molecule, based on the assumption that electrons are shared equally between bonded atoms, regardless of their electronegativity. In reality, more electronegative elements tend to attract electrons more strongly, leading to an uneven distribution of electron density. However, for the purpose of calculating formal charge, this unequal sharing is not considered.

When discussing formal charge, it is essential to differentiate between bonding electrons and nonbonding electrons. Bonding electrons are those involved in forming bonds with other atoms, while nonbonding electrons are those that do not participate in bonding.

The formula for calculating formal charge is as follows:

\[ \text{Formal Charge} = \text{Valence Electrons} - \text{Bonds} + \text{Nonbonding Electrons} \]

In this formula, the valence electrons correspond to the group number of the element in the periodic table. When counting nonbonding electrons, each electron is counted individually. By calculating the formal charge for each atom in a molecule, one can determine the net charge, which is the sum of all individual formal charges within that compound.

Understanding formal charge is crucial for predicting the stability and reactivity of molecules, as it provides insight into the distribution of electrons and the overall charge of the compound.

Formal Charge Example 1

To determine the formal charge of the nitrogen atom in the ammonia molecule (NH₃), we can use the formal charge formula:\[\text{Formal Charge (FC)} = \text{Valence Electrons} - \text{Bonds} - \text{Non-bonding Electrons}\]For nitrogen, which is in Group 5A of the periodic table, it has 5 valence electrons. In ammonia, nitrogen forms three bonds with hydrogen atoms and has two non-bonding electrons (lone pairs). Breaking it down:1. **Valence Electrons**: Nitrogen has 5 valence electrons.2. **Bonds**: Nitrogen forms 3 bonds (one with each hydrogen).3. **Non-bonding Electrons**: There are 2 non-bonding electrons.Substituting these values into the formula gives:\[\text{FC} = 5 - 3 - 2 = 0\]Thus, the formal charge of the nitrogen atom in ammonia is 0. This indicates that the nitrogen atom is neutral in the context of the ammonia molecule.

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

Answer: (A)

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

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

Answer: (A)

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

Answer: (A)

Which element within the thiocyanate ion possesses a negative charge?

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

Answer: (B)

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.