Problem 101 - Chemistry Matter and Change

Question

Draw three resonance structures for the polyatomiion \(\mathrm{CO}_{3}^{2-} .\)

Step-by-Step Solution

Answer

The three resonance structures for the carbonate ion, CO₃²⁻, are as follows: 1. Structure 1: Double bond between the Central Carbon atom and Oxygen 1, single bonds with Oxygen 2 and Oxygen 3. 2. Structure 2: Double bond between the Central Carbon atom and Oxygen 2, single bonds with Oxygen 1 and Oxygen 3. 3. Structure 3: Double bond between the Central Carbon atom and Oxygen 3, single bonds with Oxygen 1 and Oxygen 2. These structures represent the delocalized nature of electrons, as the double bond can exist between the Carbon atom and any of the three Oxygen atoms.

Step 1: Calculate the total number of valence electrons

To find the total number of valence electrons in carbonate ion, CO₃²⁻, each Carbon atom contributes 4 electrons (since it is from Group 4), 6 electrons by each Oxygen atom (since they are from Group 6), and 2 extra electrons due to the 2 negative charges on the ion. Total valence electrons = (1 Carbon atom × 4 electrons) + (3 Oxygen atoms × 6 electrons) + 2 extra electrons = 24 electrons

Step 2: Draw a skeletal structure for the ion

Place the Carbon atom in the center and the three Oxygen atoms surrounding it. This structure forms a resonance hybrid of carbonate ion since the double bond in this ion can be located between the central Carbon atom and any of the three Oxygen atoms.

Step 3: Distribute the valence electrons

Distribute the 24 valence electrons among the atoms to satisfy the octet rule for all atoms. Carbon should have 8 valence electrons around it (following the octet rule), and each of the three Oxygen atoms should also have 8 valence electrons, achieving a full valence shell for each atom. 1. Place a single bond between the central Carbon atom and each of the three Oxygen atoms. 2. Each single bond counts for 2 electrons. Currently, the structure has used 6 electrons (each bond containing 2 electrons). Remaining valence electrons: 24 - 6 = 18 electrons. 3. Complete the octet for the three Oxygen atoms by placing the remaining electron pairs (6 pairs per oxygen atom). After this step, there is a total of 6 valence electrons on Carbon and 8 valence electrons on each Oxygen atom. 4. To complete the octet for Carbon, move one lone pair from an Oxygen atom to form a double bond with the Carbon atom. This will create the first resonance structure.

Step 4: Draw all three resonance structures

Now, we draw all three resonance structures for the carbonate ion, CO₃²⁻. To signify resonance, place a double-headed arrow between the structures. 1. Structure 1: Double bond between the Central Carbon atom and Oxygen 1, single bonds with Oxygen 2 and Oxygen 3. 2. Structure 2: Double bond between the Central Carbon atom and Oxygen 2, single bonds with Oxygen 1 and Oxygen 3. 3. Structure 3: Double bond between the Central Carbon atom and Oxygen 3, single bonds with Oxygen 1 and Oxygen 2. These three structures are the resonance structures for the carbonate ion, CO₃²⁻. They represent the delocalized nature of electrons, as the double bond can exist between the Carbon atom and any of the three Oxygen atoms.

Key Concepts

Carbonate Ion

The carbonate ion, \(\text{CO}_3^{2-}\),is a polyatomic ion that plays an essential role in various chemical compounds like limestone and baking soda. It consists of a central carbon atom and three oxygen atoms arranged around it.- The carbonate ion carries a -2 charge, due to the presence of two extra electrons.- These extra electrons contribute to the formation of the ion itself.- In solutions, carbonate ions can participate in acid-base reactions, often acting as a base.Understanding the structure of the carbonate ion is crucial because it helps explain how the ion interacts in chemical environments. The presence of resonance structures illustrates the flexibility in electron distribution, allowing us to visualize its stability and reactivity in different chemical reactions.

Octet Rule

The octet rule is a fundamental concept in chemistry that helps us understand how atoms form stable molecules. It states that atoms tend to bond in such a way that each atom has eight electrons in its valence shell, achieving a noble gas configuration. - For the carbonate ion, the central carbon atom and each of the three surrounding oxygen atoms aim to satisfy this rule. - Each oxygen initially forms a single bond with carbon, attempting to achieve a full outer shell. The carbonate ion’s resonance structures demonstrate how electron pairs can be rearranged to fulfill the octet rule for all atoms involved. In the resonance structures, a lone electron pair from oxygen forms a double bond with carbon, helping carbon achieve a full octet. This sharing of electrons takes place in all three resonance forms, ensuring that the octet rule is met uniformly.

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. They play a critical role in chemical bonding since they are involved in forming bonds between atoms. - In the carbonate ion, carbon has 4 valence electrons, and each oxygen atom has 6 valence electrons. - Adding the two negative charges gives the ion a sum of 24 valence electrons to distribute. These electrons are allocated among the atoms to satisfy the requirements of the octet rule. Initially, single bonds between carbon and each oxygen atom use up 6 electrons. The remaining electrons form lone pairs to complete the octet for each oxygen. By moving one lone pair from an oxygen to form a double bond with carbon, the electron distribution optimizes stability and satisfies electron demands for all atoms involved.

Lewis Structures

Lewis structures provide a diagrammatic way to represent the valence electrons around atoms in a molecule. They illustrate how atoms share or exchange electrons to achieve stability.- In constructing a Lewis structure for the carbonate ion, carbon is placed at the center with oxygen atoms surrounding it in a triangular arrangement.Lewis structures utilize dots and lines:

• Dots represent the valence electrons.
• Lines depict shared electron pairs, indicating covalent bonds.

For the \(\text{CO}_3^{2-}\) ion, the illustrations show

multiple resonance forms where the double bond shifts between oxygen atoms connected to carbon.

All forms equally contribute to the actual structure of the ion.

These visual representations help us understand how electrons are distributed and shared among atoms in various molecules, including the carbonate ion.