What is the lewis structure for #SO_2#?

1 Answer
Dec 21, 2013

Here are the steps I follow when drawing a Lewis structure.

Explanation:

1. Decide which is the central atom in the structure. That will normally be the least electronegative atom (#"S"#).

2. Draw a skeleton structure in which the other atoms are single-bonded to the central atom: #"O-S-O"#.

3. Draw a trial structure by putting electron pairs around every atom until each gets an octet. In this editor, I will have to write it as ::Ö-S(::)-Ö::

4. Count the valence electrons in your trial structure (20).

5. Now count the valence electrons you actually have available. #"1 S + 2 O = 1×6 + 2×6 = 18"#. The trial structure has two extra electrons.

6. Draw a new trial structure, this time inserting one double bond for each extra pair of electrons:

#"O=S-O"# and #"O-S=O"#

7. As before, add valence electrons to give each atom an octet:

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8. Calculate the formal charge on each atom.

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9. We see that some of the atoms have formal charges. The “best” Lewis structure is one in which has the fewest formal charges. We can generate a structure with zero formal charges if we move a lone pair from the single-bonded #"O"# to make a double bond to the #"S"#.

This gives us a third possibility:

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We now have a structure in which #"S"# has ten valence electrons. However, that’s OK, because #"S"# can “expand” its octet.

We have three different structures, differing ONLY in the locations of the electrons. We say that these are resonance structures of #"SO"_2#.

The actual structure of #"SO"_2# is a resonance hybrid of all three structures.