Question

12 mark for review
$ddot{\text{o}}=\text{c}=ddot{\text{o}}$ \t $ddot{\text{o}}-\text{c}equiv\text{o}:$
diagram 1 \t diagram 2
which of the lewis diagrams shown above is the more likely structure of $\text{co}_2$ and why?
a diagram 1, because all the atoms have a formal charge of 0.
b diagram 1, because double bonds are stronger than triple bonds.
c diagram 2, because all the atoms have a formal charge of 0.
d diagram 2, because triple bonds are stronger than double bonds.

Explanation:

To determine the more likely Lewis structure of \( \ce{CO_2} \), we calculate the formal charges for each atom in both diagrams. The formula for formal charge is \( \text{Formal Charge} = \text{Valence Electrons} - \text{Non - bonding Electrons} - \frac{1}{2}\text{Bonding Electrons} \).

For Diagram 1 (\( \ce{O=C=O} \)):

• Oxygen atoms: Valence electrons for O is 6. Non - bonding electrons: 4 (the lone pairs), bonding electrons: 4 (from the double bond).

\( \text{Formal Charge} = 6 - 4-\frac{4}{2}=6 - 4 - 2 = 0 \)

• Carbon atom: Valence electrons for C is 4. Non - bonding electrons: 0, bonding electrons: 8 (from two double bonds).

\( \text{Formal Charge}=4 - 0-\frac{8}{2}=4 - 0 - 4 = 0 \)

For Diagram 2 (\( \ce{:O - C\equiv O:} \)):

• Left - hand O atom: Valence electrons = 6, non - bonding electrons = 6, bonding electrons = 2 (from the single bond).

\( \text{Formal Charge}=6 - 6-\frac{2}{2}=6 - 6 - 1=- 1 \)

• Carbon atom: Valence electrons = 4, non - bonding electrons = 0, bonding electrons = 8 (from one single and one triple bond).

\( \text{Formal Charge}=4 - 0-\frac{8}{2}=4 - 0 - 4 = 0 \)

• Right - hand O atom: Valence electrons = 6, non - bonding electrons = 2, bonding electrons = 6 (from the triple bond).

\( \text{Formal Charge}=6 - 2-\frac{6}{2}=6 - 2 - 3 = + 1 \)

A Lewis structure is more stable when the formal charges of the atoms are as close to zero as possible. In Diagram 1, all atoms have a formal charge of 0, while in Diagram 2, the O atoms have non - zero formal charges. Also, the statement in option B is incorrect because triple bonds are stronger than double bonds. Option C is wrong as we saw Diagram 2 has non - zero formal charges. Option D is incorrect because the stability of the Lewis structure is mainly determined by formal charges (and octet rule, which is satisfied in both), not just bond strength, and also Diagram 2 has non - zero formal charges.

Answer:

A. Diagram 1, because all the atoms have a formal charge of 0.