Problem 46 - Chemistry Matter and Change

Question

Challenge Draw the Lewis resonance structure for the ion \(\mathrm{SO}_{3}^{2-}\) .

Step-by-Step Solution

Answer

The Lewis structure of \( \mathrm{SO}_{3}^{2-} \) has three resonance forms with one double bond between sulfur and oxygen, shifting among the three oxygens.

Step 1: Count Total Valence Electrons

To draw the Lewis structure, we first need to determine the total number of valence electrons. Sulfur (S) has 6 valence electrons, oxygen (O) has 6 each, and there are 3 oxygen atoms. The SO_3^{2-} ion has a charge of -2, meaning we add 2 extra electrons. Thus, the total is calculated as follows: \(6 + 3 \times 6 + 2 = 26\) valence electrons.

Step 2: Draw the Skeleton Structure

With sulfur as the central atom, attach three oxygen atoms around it. This forms the skeleton structure. Connect each oxygen atom to the sulfur with a single bond. Single bonds account for 2 electrons each, using up \(6\) of those \(26\) electrons.

Step 3: Complete the Octet on Outer Atoms

Fill the octet for each of the three oxygen atoms by adding lone pairs. Each oxygen needs 8 electrons around it (including bond pairs), and each single bond already accounts for 2 electrons. Therefore, each oxygen needs 6 more electrons, which total \(3 imes 6 = 18\) electrons.

Step 4: Place Remaining Electrons on Central Atom

After filling the octets for oxygen, we have used \(24\) electrons. The \(26 - 24 = 2\) remaining electrons are placed as a lone pair on the sulfur atom, completing the octet rule for all atoms.

Step 5: Check and Add Resonance Structures

Analyse the structure created. In this case, the structure can have resonance as double bonds can shift among the oxygen atoms. Move one of the lone pairs from one oxygen to form a double bond with sulfur, redistributing electrons while still maintaining total count. Repeat this for other oxygens to show possible equivalent forms.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a critical role in forming chemical bonds. These electrons determine how an element will interact with others. For the ion \(\mathrm{SO}_{3}^{2-}\), valence electrons dictate the Lewis structure we draw.

• Sulfur (S) sits in group 16 of the periodic table. It has 6 valence electrons.
• Each oxygen (O) atom also contributes 6 valence electrons, and since there are 3 oxygen atoms, that sums up to 18 electrons from oxygen alone.
• The negative charge of -2 on the \(\mathrm{SO}_{3}^{2-}\) ion indicates that we have 2 additional electrons to account for, leading to a total of 26 valence electrons.

Understanding how valence electrons contribute to forming bonds is crucial. When drawing the Lewis structure, these electrons help us determine the bond formation and lone pairs involved. In this case, we use them to connect sulfur and oxygen atoms, ensuring we maintain the accurate electronic configuration.

Resonance Structures

Resonance structures are multiple ways to showcase the delocalization of electrons in a molecule. These help represent the possible configurations that molecules like \(\mathrm{SO}_{3}^{2-}\) could theoretically have.

• In our \(\mathrm{SO}_{3}^{2-}\) ion, by moving electrons within the molecule, we can represent different structures.
• It's important to understand that resonance forms aren't actual structures. Rather, the real structure is a hybrid of all possible forms, best represented by each one.
• In this exercise, we see the shifting of a lone pair from oxygen to form a double bond with sulfur. This is done while ensuring the total number of valence electrons remains unchanged.

The beauty of resonance is in showing the flexibility and delocalization of electrons, providing a deeper insight into a molecule's true electronic structure.

Octet Rule

The octet rule is a foundational concept in chemistry. It states that atoms tend to form bonds in such a way that each atom ends up with eight electrons in its valence shell, achieving a noble gas configuration.

• For \(\mathrm{SO}_{3}^{2-}\), this means arranging electrons so that each atom, especially oxygen, has a complete set of 8 electrons around it.
• Sulfur, as the central atom, forms three single bonds initially. Each bond has 2 electrons, making a total of 6 electrons involved with the outer atoms.
• Lone pairs are used to ensure that each oxygen atom satisfies the octet requirement. Oxygen needs 6 more electrons to complete its octet after accounting for the bond electrons.

It's worth noting that while the octet rule is a useful guideline, there are exceptions, particularly for sulfur. As a molecule maintains resonance structures, the perceived "octet" might shift. The key takeaway is achieving a stable configuration through electron sharing.

Sulfur Trioxide Ion

The sulfur trioxide ion, \(\mathrm{SO}_{3}^{2-}\), is a fascinating species in chemistry, showcasing multiple important principles in bonding and structure.

• The ion's structure is based around sulfur acting as a central atom connected to three oxygen atoms.
• This configuration involves single and double bonds, which are interchangeable due to resonance.
• The \(\mathrm{SO}_{3}^{2-}\) ion is an example of how more than one resonance structure can exist, emphasizing sulfur's ability to expand its octet due to available d-orbitals.

Understanding the \(\mathrm{SO}_{3}^{2-}\) ion requires a balanced view of traditional rules, like the octet rule, while recognizing the flexibility seen in advanced concepts like resonance. This blend showcases the beauty of chemical bonding and molecular structure.