Draw a Lewis structure for the bicarbonate ion, HCO3−(sodium bicarbonate is the main ingredient of baking soda). Show all resonance forms and nonzero formal charges. What is the bond order of each C-O bond?
The Correct Answer and Explanation is:
To draw the Lewis structure for the bicarbonate ion (HCO₃⁻), we follow these steps:
- Count Valence Electrons:
- Carbon (C) has 4 valence electrons.
- Oxygen (O) has 6 valence electrons, and there are three O atoms.
- Hydrogen (H) has 1 valence electron.
- The bicarbonate ion has a -1 charge, which adds 1 extra electron.
- Total = (4 + (3 \times 6) + 1 + 1 = 22) valence electrons.
- Draw the Skeletal Structure:
- Carbon is the central atom, bonded to one hydrogen atom and three oxygen atoms.
- Distribute Electrons:
- Initially, connect the C atom to one O with a double bond and the other two O atoms with single bonds.
- Place the hydrogen atom bonded to one of the singly bonded oxygen atoms.
- Distribute the remaining electrons to complete the octets of the oxygen atoms.
- Formal Charges:
- For the central carbon atom: (4 – (0 + 0 + 0) = 4) (4 is for its valence electrons, no lone pairs, and it shares 4 electrons).
- For the doubly bonded oxygen: (6 – (4 + 0) = 2) (6 for valence, 4 for bonding pairs).
- For each singly bonded oxygen: (6 – (2 + 1) = 3).
- For hydrogen: (1 – (0 + 0) = 1).
- The sum of formal charges: (-1) from the overall ion and (0) from the hydrogen.
- Resonance Forms:
- The bicarbonate ion exhibits resonance, where the double bond can be between carbon and any of the three oxygen atoms. Therefore, three resonance structures can be drawn.
Bond Order
- The bond order for each C-O bond in the bicarbonate ion can be calculated as follows:
- For the double-bonded C=O bond, the bond order is (2).
- For the single-bonded C-O bonds, the bond order is (1).
Given the resonance structures, each C-O bond effectively has a bond order of (1.33) (the average of one double bond and two single bonds).
Summary
The bicarbonate ion (HCO₃⁻) exhibits resonance with three major forms and formal charges indicating that one oxygen has a -1 charge, while the carbon maintains a neutral charge. The bond order reflects that the C-O bonds are not purely single or double but a mix due to resonance, leading to an average bond order of 1.33.
Lewis Structure Diagram
Here’s the Lewis structure representation for clarity:
O
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H-C-O⁻
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O⁻This structure can shift between different resonance forms where the double bond is shared among the three oxygen atoms.