Formal Charge

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1 of 2 videos by Khan Academy - Organic Chemistry

Key Questions

  • Answer:

    Formal charge is the charge left on the central atom when all the bonding pairs (of electrons) are removed sequentially.


    Let's consider the simple case of ammonia, #NH_3#, versus its ammonium salt, #NH_4^+#. Now ammonia is a neutral molecule, and there is a non-bonding pair, a lone pair, of electrons localized to the nitrogen centre. Its reaction with #H^+# is very simply represented:

    #NH_3 + H^+ rarr NH_4^+#

    The ammonium is now quaternized. Why? Well, any chemical reaction conserves mass and charge, and this one does as well. But why do we write ammonium as #NH_4^+#, with a formal positive charge on the nitrogen nucleus?

    The nitrogen in ammonia, #NH_3# is neutral because it shares 3 electrons from the 6 electrons that comprise the #N-H# bonds, and gets a full contribution from its lone pair, i.e. 5 electrons + 2 inner electrons electrostatically balance the 7 protons in the nucleus of a nitrogen atom. When #N# is quaternized as ammonium, #NH_4^+#, it is conceived to have a #1/2# share only of the 8 electrons of the #N-H# bonds, #4 e# in total, and is therefore written as #NH_4^+#, with the positive charge formally associated with the nitrogen atom.

  • Answer:

    We can use formal charges to know if we drew the best Lewis structure of a compound.


    Some Lewis structures look satisfy the Octet rule and all that but sometimes, there are better structures to be drawn and formal charges can help you with that. If the formal charge of all the elements in a compound is equal to the charge of the compound, that means the structure is correct and it's the best structure for that compound.

  • The formula for calculating the formal charge on an atom is simple.

    Formal charge = [# of valence electrons] – [electrons in lone pairs + 1/2 the number of bonding electrons]

    Since the number of bonding electrons divided by 2 is equal to the number of bonds surrounding the atom, this formula can be shortened to:
    Formal Charge = [# of valence electrons on atom] – [non-bonded electrons + number of bonds].

    Let's look at an example. Take the compound #BH^4#, or tetrahydrdoborate. Boron, #(B)# has 3 valence electrons, zero non-bonded electrons, and 4 bonds around it. This means that the formula becomes #3-(0+4)#, giving an answer of #-1#.

    Next, let's look at the hydrogen atoms in #BH^4#. Hydrogen has one valence electron, zero non bonded electrons, and one bond. So the formal charge of hydrogen in #BH^4# is #1-(0+1)#, which gives a formal charge of 0.