What is the lewis structure for ##SO_2##?

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

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 in your trial structure (20).

5. Now count the 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:

8. Calculate the formal charge on each atom.

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:

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.