Step 1: The total number of valence electrons of the atoms is 23 plus one for the negative charge, giving a total of 24.
Step 2: We can deduce the skeletal structure of the bicarbonate ion by realizing that C is less electronegative than O. Therefore, it is most likely to occupy a central position as follows:
Step 3: The number of electrons remaining is `(24-8=)16`.
Step 4: The number of electrons needed to complete all octets is `[(2xx6)+2+4=]18`.
Step 5: One additional bond is needed. It is best placed between the C atom and either of the O atoms not bonded to H:
Step 6: Completing the octets gives the structure:
which accounts for the required 24 electrons and indicates the negative charge of the species.
