Formal Charges and How To Calculate Them


Formal Charges and How To Calculate Them

** Many Lewis structures are incomplete until we decide whether any of their atoms have a formal charge.

 ** Calculating the formal charge on an atom in a Lewis structure is simply a bookkeeping method for its valence electrons.
** First, we examine each atom and, using the periodic table, we determine how many valence electrons it would have if it were an atom not bonded to any other atoms. This is equal to the group number of the atom in the periodic table. For hydrogen this number equals 1, for carbon it equals 4, for nitrogen it equals 5, and for oxygen it equals 6.
** Next, we examine the atom in the Lewis structure and we assign the valence electrons in the following way:
** We assign to each atom half of the electrons it is sharing with another atom and all of its unshared (lone) electron pairs.
** Then we do the following calculation for the atom:
Formal charge = number of valence electrons – 1/2 number of shared electrons – number of unshared electrons


where F is the formal charge, Z is the group number of the element, S equals the number of shared electrons, and U is the number of unshared electrons.
** It is important to note, too, that the arithmetic sum of all the formal charges in a molecule or ion will equal the overall charge on the molecule or ion.

Examples for calculating formal Charges 

Let us consider several examples showing how this is done.

The Ammonium Ion (NH4+)

** As we see below, the ammonium ion has no unshared electron pairs.
** We divide all of the electrons in bonds equally between the atoms that share them. Thus, each hydrogen is assigned one electron.
** We subtract this from one (the number of valence electrons in a hydrogen atom) to give each hydrogen atom a formal charge of zero.

** The nitrogen atom is assigned four electrons (one from each bond). We subtract four from five (the number of valence electrons in a nitrogen atom) to give the nitrogen a formal charge of +1.

The Nitrate Ion (NO3)

** Let us next consider the nitrate ion (NO3), an ion that has oxygen atoms with unshared electron pairs.

** Here we find that the nitrogen atom has a formal charge of +1, that two oxygen atoms have formal charges of -1, and that one oxygen has a formal charge equal to 0.

Water and Ammonia

** The sum of the formal charges on each atom making up a molecule must be zero.

** Consider the following examples:


Practice Problem(1): Write a Lewis structure for each of the following negative ions, and assign the formal negative charge to the correct atom:

A Summary of Formal Charges

** With this background, it should now be clear that each time an oxygen atom of the type:


appears in a molecule or ion, it will have a formal charge of -1, and that each time an oxygen atom of the type :



 appears, it will have a formal charge of 0.

** Similarly,


** These and other common structures are summarized in this Table:



Practice Problem(2): Assign the proper formal charge to the colored atom in each of the following structures:



Reference: Organic chemistry / T.W. Graham Solomons , Craig B.Fryhle , Scott A.snyder , / ( eleventh edition) / 2014.


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