Animated Solution for Chemistry - Chemical Bonding and Molecular Structure: Identify the species having one $\pi$ - bond and maximum number of canonical forms from the following

When tackling a multiple-choice question in chemistry, the first step is to act like a detective and carefully read the constraints. This particular problem sets up a fascinating dual-condition hunt. We are looking for a specific chemical species that satisfies two strict rules simultaneously:

1. The Pi-Bond Rule: The molecule or ion must possess exactly one $\pi$-bond. Not zero, not two, but exactly one. 2. The Resonance Rule: Among the candidates that pass the first rule, our target must exhibit the maximum number of canonical forms (resonance structures).

Let's put our candidates under the microscope and see how they hold up against these rules.

A Structural Breakdown

1. Sulfur Trioxide (${\text{SO}}_3$) Let's draw the Lewis structure for ${\text{SO}}_3$. The central sulfur atom is bonded to three oxygen atoms. In one of its major contributing resonance structures, sulfur forms double bonds with two oxygen atoms and a single bond with the third.

Because each double bond consists of one $\sigma$-bond and one $\pi$-bond, this structure has two $\pi$-bonds. Since the single bond can be located on any of the three oxygen atoms, it has three canonical forms. However, because it has two $\pi$-bonds, it immediately fails our first condition.

2. Oxygen Molecule (${\text{O}}_2$) The oxygen molecule is a simple diatomic species. Its Lewis structure is straightforward: $\text{O}=\text{O}$.

This molecule has exactly one $\pi$-bond, so it passes the first test! But what about resonance? Because there are no other equivalent positions for the double bond to move to, it has only one canonical form. It's a contender, but we need to see if we can find a species with more resonance structures.

3. Sulfur Dioxide (${\text{SO}}_2$) Moving on to ${\text{SO}}_2$, the central sulfur atom is bonded to two oxygen atoms. To satisfy valencies, it forms a double bond with one oxygen and a single bond with the other.

This gives us exactly one $\pi$-bond. The double bond can resonate between the two oxygen atoms, meaning it has two canonical forms. This is better than ${\text{O}}_2$, but is it the absolute maximum?

4. Carbonate Ion (${\text{CO}}_3^{2-}$) Finally, let's examine the carbonate ion. The central carbon atom is bonded to three oxygen atoms. It forms a double bond with one oxygen and single bonds with the other two (which carry the negative charges).

This structure has exactly one $\pi$-bond, passing our first rule with flying colors. Now, because that single double bond can be shared equally among any of the three oxygen atoms, the carbonate ion boasts three canonical forms.

Let's summarize our findings: ${\text{SO}}_3$: $2$ $\pi$-bonds, $3$ canonical forms (Fails Rule 1) ${\text{O}}_2$: $1$ $\pi$-bond, $1$ canonical form ${\text{SO}}_2$: $1$ $\pi$-bond, $2$ canonical forms ${\text{CO}}_3^{2-}$: $1$ $\pi$-bond, $3$ canonical forms

Among the species that have exactly one $\pi$-bond (${\text{O}}_2$, ${\text{SO}}_2$, and ${\text{CO}}_3^{2-}$), the carbonate ion (${\text{CO}}_3^{2-}$) clearly has the maximum number of canonical forms. Therefore, it is the perfect match for our dual-condition hunt!