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

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Jun 20, 2015


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:

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

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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.

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

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