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

1 Answer
Sep 3, 2016

Answer:

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.