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Formal Charge

The concept of formal charge is based on the assumption that the electrons involved in the bonds are equally shared between the involved atoms. As you have already seen, this is not exactly the case. But for simplicity and from an analytical point of view, we assume that the electrons are equally distributed.

Formal charge of the atoms is the charge of those atoms in a formula, under the assumption that the electrons in the bonds are equally distributed between the atoms that contain the bond.


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There are different ways to find formal charges. One way is mentioned here. You can either choose to do it this way, or whichever way you are comfortable with.

Example 5-2

Find the formal charges of all the atoms in thionyl chloride (SOCl2).



The Lewis structure of thionyl chloride is written below:


image\24564 Ch 5.jpg


Using the formal charge formula, it is just a matter of plugging in the numbers. Both oxygen and sulfur belong to Group VI. Chlorine belongs to Group VII. Sulfur has 3 bonds and 2 unshared electrons. Oxygen has 1 bond and 6 unshared electrons. Chlorine has 1 bond and 6 unshared electrons.


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The formal charge of sulfur = 6 – 3 – 2 = +1

The formal charge of oxygen = 6 – 1 – 6 = –1

The formal charge of chlorine = 7 – 1 – 6 = 0


Notice that the net charge of the molecule is zero, as expected, since it is a molecule and not an ion.


Problem 5-1
Which of the following represents the formal charge of sulfur in sulfuric acid (H2SO4)?
  1. –1
  2. +1
  3. –2
  4. +2

If you draw the structure, you'll see that sulfur is the central atom, and has 2 single bonds to the 2 oxygen atoms, and 2 single bonds to the OH groups. The answer is +2.

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