# What is the best Lewis structure for HCO_3^-? Why is it best using formal charges? How do you rank bond lengths from longest to shortest for each bond in the molecule?

Jun 18, 2017

Well, the Lewis structure of the parent acid is........

.........$\left(O =\right) C {\left(- O H\right)}_{2}$,

#### Explanation:

.........$\left(O =\right) C {\left(- O H\right)}_{2}$, and so the structure of the base is...

$\left(O =\right) C \left(- O H\right) \left(- {O}^{-}\right)$ else $\left(O =\right) C {\left(- O\right)}_{2}^{-}$

In $C {O}_{3}^{2 -}$ there are $3 \times 6 \left(O\right) + 4 \left(C\right) + 2 \left(\text{negative charges}\right) = 24$, i.e. $12 \cdot \text{electrons pairs}$ to distribute around 3 centres.

And given $3 \times 8 + 6 = 30$ nuclear charges, i.e. the positively charged protons in the carbon and oxygen nuclei, and the 8 inner core (i.e. $1 {s}^{2}$ electrons on each atom), we has 32 electronic charges BUT ONLY 30 protonic, nuclear charges. The difference, ${\text{^(-)32+}}^{+} 30 = - 2$ clearly, and this carbonate has a formal charge of $- 2$.

We would expect the $C = O$ bonds to be the shortest, however, we would struggle to find an experimental regime which could interrogate the difference in $C - O$ bonds lengths.........Simple ideas of resonance stabilization could account for approx. equal $C - O$ bond lengths, somewhat shorter than an actual $C - O$ bond, i.e. $< 1.40 \times {10}^{-} 10 \cdot m$.