How can I draw two equivalent resonance structures for the formate ion, #HCO_2^-#?

Question

22966 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2015-06-20T00:01:11

Use a systematic procedure to draw Lewis structures for the ion.

Here are the steps that 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 (C).

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

enter image source here

3. Draw a trial structure by putting electron pairs around every atom until each gets an octet.

enter image source here

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

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

#1 H + 1 C + 2 O = 1×1 ++ 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.

You have two possible choices for the double bond.

enter image source here

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

enter image source here

8. Calculate the formal charge on each atom.

enter image source here

And these are the two resonance structures for the formate ion..