Question

The oxalate ion, \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\), has the skeleton structure (a) Complete the Lewis structure of this ion. (b) Draw three resonance forms for \(\mathrm{C}_{2} \mathrm{O}_{4}{ }^{2-}\), equivalent to the Lewis structure drawn in (a). (c) Is a resonance form of the oxalate ion?

Short answer

Short Answer: (a) The completed Lewis structure for the oxalate ion is: O=C-O-C=O .. .. (b) Three resonance forms for the oxalate ion are: Form 1: O=C-O-C=O .. .. Form 2: ..O-C=O-C=O.. Form 3: O=C-C=O ..O...O.. (c) The provided resonance form is not a valid resonance form for the oxalate ion since it doesn't match any of the valid resonance forms generated in (b).

Step-by-step solution

(a) Complete the Lewis structure of the oxalate ion

To complete the Lewis structure, we will first count the total number of valence electrons. Carbon has 4 valence electrons, each oxygen atom has 6 valence electrons, and there are 2 extra electrons for the \(2-\). So the total valence electron count is \((2\times4)+(4\times6)+2=24\) electrons. Next, we will connect the atoms with single bonds. Place the two carbon atoms in the center, bonded to each other. Then, connect each carbon atom with two oxygen atoms. This requires 6 bonds (2 electrons each), which leaves us with \(24-12=12\) electrons remaining. Now distribute the remaining electrons to the oxygen atoms to complete their octets. Each oxygen atom needs two more electrons, so we can place one lone pair (2 electrons each) on each oxygen atom, using all 12 remaining electrons. Here's the completed Lewis structure for the oxalate ion: O=C-O-C=O .. ..

(b) Draw three resonance forms for the oxalate ion

Based on the completed Lewis structure, we can find three resonance forms by rearranging double bonds and lone pairs while keeping single bonds intact. Let's draw the three resonance forms: Resonance Form 1: O=C-O-C=O .. .. Resonance Form 2: ..O-C=O-C=O.. Resonance Form 3: O=C-C=O ..O...O..

(c) Determine if the provided resonance form is valid for the oxalate ion

The provided resonance form is : Now, compare this provided resonance form to the three resonance forms we drew in (b). We can see that it doesn't match any of the valid resonance forms. Therefore, the provided resonance form is not a valid resonance form for the oxalate ion.

Key concepts

Lewis structure

The Lewis structure is a visual representation of the arrangements of atoms and electrons in a molecule or ion. For the oxalate ion, \(\text{C}_2\text{O}_4^{2-}\), forming the Lewis structure involves knowing the total number of valence electrons. Valence electrons are the electrons in the outermost shell of an atom and are crucial in bonding.
To draw the Lewis structure, start by calculating the total valence electron count. Each carbon has 4 valence electrons, each oxygen has 6, and the ion has 2 extra electrons due to its negative charge, giving us a total of 24 valence electrons. This is computed as follows:
\( (2 \times 4) + (4 \times 6) + 2 = 24 \) valence electrons.
Place the carbon atoms in the center since they form the backbone of the structure. Connect them with single bonds (each bond representing 2 electrons) to the oxygen atoms. This uses up 12 electrons for 6 bonds, leaving us with 12 electrons to distribute as lone pairs.

Distribute these electrons to complete the octet for each oxygen. Each oxygen atom already has 2 electrons from bonding, so each needs 6 more to reach the octet (total of 8 with shared electrons). The structure would feature all non-bonding pairs neatly completing the octets of oxygen and satisfying the Lewis structure requirements.

Resonance forms

Resonance forms are different ways to draw a molecule's structure without changing the placement of the atoms, only the arrangement of the electrons. This concept is crucial to understanding molecules like the oxalate ion, which can be represented in multiple ways.
The oxalate ion can have several resonance forms, showcasing the movement of electrons while maintaining single bonds between the atoms. All resonance forms equally contribute to a hybrid which captures the molecule's properties more accurately. This delocalization of electrons provides extra stability to the molecule.
For \(\text{C}_2\text{O}_4^{2-}\), there are three main resonance forms. They involve the double bonds shifting between different oxygen atoms, demonstrating that the electrons are shared across these positions.

• Resonance Form 1: Two double bonds connect adjacent oxygen atoms to the central carbon atoms on both sides.
• Resonance Form 2: A variation where one side has a shifted double bond pair.
• Resonance Form 3: Alternative shifting, also maintaining overall electron count and structure balance.

These forms indicate how the molecule doesn't have a single fixed structure, but rather, a collection of states defining its resonance hybrid.

Valence electrons

Valence electrons play a vital role in chemical bonding and molecule formation. They are found in the outermost shell of an atom and are primarily responsible for binding atoms together. Understanding valence electrons is crucial when drawing Lewis structures and identifying resonance forms for ions.
In the oxalate ion example, knowing the number of valence electrons allows us to predict how electrons are shared or transferred during bond formation. Carbon atoms with 4 valence electrons bond to oxygen atoms each with 6 valence electrons, influencing how electrons are distributed in the ion's structure.
Charge considerations also affect valence electron count. For the oxalate ion, \(\text{C}_2\text{O}_4^{2-}\), the 2- charge indicates an addition of 2 electrons to the total count. This important detail ensures we correctly tally the 24 total valence electrons required for bonding and completing octets.

• Bonding: Connect carbon atoms and oxygen atoms with single bonds.
• Non-bonding: Distribute the remaining electrons as lone pairs to complete the octets, focusing first on more electronegative atoms like oxygen.

By understanding valence electrons, one can correctly construct Lewis structures and resonance forms, capturing the chemical behavior of ions and molecules effectively.