Question

How do you draw the Lewis structure for `SeO_2`. What is the electron geometry around the central atom?

Answer 1

WARNING! Long answer.

Explanation:

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 (`"Se"`).

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

`"O-Se-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

`:stackrel(. .)("O")-stackrel(. .)("Se")-stackrel(. .)("O"":`
`color(white)(ll)stackrel(. .color(white)(mm). .color(white)(mmll). .)(color(white)(mmmmml))`

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

5. Now count the valence electrons you actually have available.

`"1 Se + 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=Se-O"` and `"O-Se=O"`

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

`:stackrel(. .)("O")="Se"-stackrel(. .)("O"":`
`color(white)(ll)stackrel(. .color(white)(mm). .color(white)(mmll). .)(color(white)(mmmmml))`

and

`:stackrel(. .)("O")-"Se"=stackrel(. .)("O"":`
`color(white)(ll)stackrel(. .color(white)(mm). .color(white)(mmll). .)(color(white)(mmmmml))`

8. Calculate the formal charge on each atom.

`:stackrel(. .)("O")=stackrel(+1)("Se")-stackrel(. .)("O"):^"-1"`
`color(white)(ll)stackrel(. .color(white)(mm). .color(white)(mmll). .)(color(white)(mmmmml))`

and

`""^"-1":stackrel(. .)("O")-stackrel(+1)("Se")=stackrel(. .)("O"":`
`color(white)(mll)stackrel(. .color(white)(mm). .color(white)(mmll). .)(color(white)(mmmmml))`

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:

`:stackrel(. .)("O")=stackrel(. .)("Se")=stackrel(. .)("O"):`

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 `"SeO"_2`.

The actual structure of `"SeO"_2` is a resonance hybrid of all three structures.

In all three structures, there are three electron domains about the `"Se"` atom: the lone pair and the bonds on either side.

The electron geometry around `"Se"` is trigonal planar.