Sometimes when writing a
Lewis structure you come across two different ways to write the molecule,
both which look fine. In this case, you should use formal charge to
decide which structure is correct for the molecule.
The formal charge is the difference in the number of valence electrons in the
atom and the number of valence electrons in the Lewis structure. The equation
for the formal charge of any atom in a Lewis structure is
- Cf = Ev - (Eu +
To decide if a given structure is correct, check
the formal charge on some atoms in all possible structures. In general the most
likely Lewis structure has:
- Cf is the formal charge
- Ev is the number of valence electrons in the bare atom
- Eu is the number of electrons in lone pairs on the atom in the
- Ep is the number of electrons in bonded pairs on the atom in
the Lewis structure
- all formal charges as close to zero as possible
- negative formal charges on electronegative atoms like halogens or oxygen.
For example, consider the methanol molecule CH3OH. This can be
written two different ways:
In both cases the octet rule is
satisfied for all of the atoms in the structure. Which is correct?
To decide, work out the formal charges on the carbon and oxygen in each
molecule. Carbon has 4 valence electrons in the base atom, oxygen 6. For the
- The carbon has four bonds, each worth 2 electrons, for a total of eight.
It has no lone pairs. Thus, Cf = 4 - (0 + 1/2*8) =0
- The oxygen has two bonds, each worth 2 electrons, for a total of four. It
has two lone pairs. Thus, Cf = 6 - (4 + 1/2*4) =0
leftmost structure has the formal charges closer to zero, and thus is probably
the correct structure.
- The carbon has three bonds, each worth 2 electrons, for a total of six. It
has one lone pair. Thus, Cf = 4 - (2 + 1/2*6) = -1
- The oxygen has three bonds, each worth 2 electrons, for a total of six. It
has one lone pair. Thus, Cf = 6 - (2 + 1/2*6) = +1
Example: There are two possible Lewis structures for the sulfurous
acid molecule, H2SO3. Which is probably correct?
Solution: The difference between the two is the double bond in the
leftmost one vs. the single bond + three lone pairs on the oxygen on the right.
Work out the formal charge on each: the oxygen and sulfur both have six valence
electrons. for the left structure
For the right
- The oxygen has two bonds (total 4 electrons) and two lone pairs (total 4
electrons). Cf = 6 - (4 + 1/2*4) = 0
- The sulfur has four bonds (total 8 electrons) and one lone pair (total 2
electrons). Cf = 6 - (2 + 1/2*8) = 0
Thus, the left
structure is probably correct
- The oxygen has one bond (total 2 electrons) and three lone pairs (total 6
electrons). Cf = 6 - (6 + 1/2*2) = -1
- The sulfur has three bonds (total 6 electrons) and one lone pair (total 2
electrons). Cf = 6 - (2 + 1/2*6) = +1