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3.1.3: Formal Charge

  • Page ID
    167330
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    The formal charge of an atom in a molecule is the hypothetical charge the atom would have if we could redistribute the electrons in the bonds evenly between the atoms. Another way of saying this is that formal charge results when we take the number of valence electrons of a neutral atom, subtract the nonbonding electrons, and then subtract the number of bonds connected to that atom in the Lewis structure.

    Calculating Formal Charges

    We calculate formal charge of an atom in a molecule or polyatomic ions as follows:

    Formal Charge = (valence electrons of the "free" element) - (unshared electrons) - (bonds).

    We can double-check formal charge calculations by determining the sum of the formal charges for the whole structure. The sum of the formal charges of all atoms in a molecule must be zero; the sum of the formal charges in an ion should equal the charge of the ion.

    We must remember that the formal charge calculated for an atom is not the actual charge of the atom in the molecule. Formal charge is only a useful bookkeeping procedure; it does not indicate the presence of actual charges.

    Calculating Formal Charge from Lewis Structures

    Assign formal charges to each atom in the interhalogen ion \(\ce{ICl4-}\).

    Solution

    We divide the bonding electron pairs equally for all \(\ce{I–Cl}\) bonds:

    imageedit_2_8215662475.png

    We assign lone pairs of electrons to their atoms. Each \(\ce{Cl}\) atom now has seven electrons assigned to it, and the I atom has eight.

    Subtract this number from the number of valence electrons for the neutral atom:

    • I: 7 – 8 = –1
    • Cl: 7 – 7 = 0

    The sum of the formal charges of all the atoms equals –1, which is identical to the charge of the ion (–1).

    Exercise \(\PageIndex{1}\)

    Calculate the formal charge for each atom in the carbon monoxide molecule:

    imageedit_9_8783034466.png

    Answer

    C −1, O +1

    Example: Calculating Formal Charge from Lewis Structures

    Assign formal charges to each atom in the interhalogen molecule \(\ce{BrCl3}\).

    Solution

    Assign one of the electrons in each Br–Cl bond to the Br atom and one to the Cl atom in that bond:

    imageedit_6_8383188586.png

    Assign the lone pairs to their atom. Now each Cl atom has seven electrons and the Br atom has seven electrons.

    Subtract this number from the number of valence electrons for the neutral atom. This gives the formal charge:

    • Br: 7 – 7 = 0
    • Cl: 7 – 7 = 0

    All atoms in \(\ce{BrCl3}\) have a formal charge of zero, and the sum of the formal charges totals zero, as it must in a neutral molecule.

    Exercise \(\PageIndex{2}\)

    Determine the formal charge for each atom in \(\ce{NCl3}\).

    Answer

    N: 0; all three Cl atoms: 0

    imageedit_28_5766396641.png


    3.1.3: Formal Charge is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by OpenStax.