Problem 71 - Chemistry and Chemical Reactivity

Question

Draw the electron dot structure for the sulfite ion, \(\mathrm{SO}_{3}^{2-} .\) In aqueous solution, the ion interacts with \(\mathrm{H}^{+}\) Predict whether a \(\mathrm{H}^{+}\) ion will attach to the \(\mathrm{S}\) atom or the \(\mathrm{O}\) atom of \(\mathrm{SO}_{3}^{2-}\).

Step-by-Step Solution

Answer

The \( \mathrm{H}^{+} \) ion will attach to an oxygen atom in \( \mathrm{SO}_{3}^{2-}\).

Step 1: Determine the total number of valence electrons

To find the total number of valence electrons for the sulfite ion \( \mathrm{SO}_{3}^{2-} \), we add the valence electrons of all the atoms involved. Sulfur has 6 valence electrons, and each oxygen has 6 valence electrons. Since there are three oxygen atoms, we count these as 3 \( \times \) 6 = 18. Additionally, the ion has a charge of \( 2- \), meaning there are 2 extra electrons. Therefore, the total is 6 (S) + 18 (O) + 2 (extra) = 26 valence electrons.

Step 2: Draw the skeleton structure

Place sulfur in the center because it is the least electronegative atom compared to oxygen. Connect each oxygen atom with a single bond to the sulfur atom. Now, we have used 2 electrons per bond, so 3 bonds use 6 electrons, leaving us with 26 - 6 = 20 electrons.

Step 3: Distribute remaining electrons to fulfill octet rule

Distribute the remaining 20 electrons, starting with the oxygen atoms to fulfill the octet rule. Each oxygen atom requires 8 electrons around it. After placing 6 more electrons around each of the three oxygens (using a total of 18 electrons), each oxygen has a full octet. We are left with 2 electrons.

Step 4: Form double bonds if necessary

Place any remaining electrons on the central sulfur atom. If necessary for the central atom to fulfill its electron requirements, re-arrange bonds to form double bonds. The sulfur in the sulfite ion can expand its octet. Form one double bond with one of the oxygen atoms to better distribute the electron density. This leaves a formal structure with one sulfur-oxygen double bond and two sulfur-oxygen single bonds.

Step 5: Predict where \( \mathrm{H}^{+} \) will bind

In \( \mathrm{SO}_{3}^{2-} \), the negative charge is primarily found on the oxygen atoms rather than on sulfur. The \( \mathrm{H}^{+} \) ion, being positively charged, will be attracted to one of the negatively charged oxygen atoms in the ion. Therefore, it is more likely to attach to an oxygen atom rather than the sulfur atom.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons in an atom and are crucial in determining how atoms bond with each other. Electrons in the outermost shell are responsible for the atom's chemical behavior. For the sulfite ion, \( \mathrm{SO}_3^{2-} \), understanding valence electrons is crucial because:

• Sulfur has 6 valence electrons.
• Each oxygen atom also has 6 valence electrons.
• Since \( \mathrm{SO}_3^{2-} \) carries a 2- charge, we add two additional electrons.

Adding these together, we calculate a total of 26 valence electrons. Knowing the total number of valence electrons guides the drawing of electron dot structures, allowing us to visualize the sharing and pairing of electrons for bonding.

Octet Rule

The octet rule is a chemical guideline that explains an atom's tendency to bond by achieving an outer shell of 8 electrons, which is associated with a stable, "noble gas-like" configuration. In the electron dot structure of \( \mathrm{SO}_3^{2-} \), this rule is used to distribute electrons:

• Each oxygen aims to complete its octet by sharing electrons or standing alone with unshared pairs.
• Total electrons after single bonds are checked against the octet rule of each atom.
Following the initial distribution process, every oxygen reaches its full octet. However, sulfur, being in period 3 of the periodic table, can expand its valence shell beyond 8 electrons and thus forms a double bond with one oxygen atom. This step balances the electron count and ensures a comprehensive structure.

Sulfite Ion

The sulfite ion, \( \mathrm{SO}_3^{2-} \), is a charged polyatomic ion with a specific arrangement in its structure. It consists of one sulfur atom centrally located and bonded to three oxygen atoms. Understanding it at an atomic level involves:

• Sulfur serving as the hub due to its ability to expand its octet.
• Oxygen atoms each sharing electrons in a way that contributes to the double-negative charge.
• The charges are typically spread among the oxygens, making the ion reactive with positively charged species.

This characteristic helps in predicting which atom in the sulfite ion will interact with hydrogen ions, providing a basis for understanding reactions in aqueous solutions, such as when the \( \mathrm{H}^+ \) ion binds with one of the oxygens.

Chemical Bonding

Chemical bonding is the interaction that holds atoms together within a compound or ion. These interactions involve sharing or transferring valence electrons between atoms to achieve stability. In the \( \mathrm{SO}_3^{2-} \) ion, chemical bonding takes form through:

• Single and double bonds between sulfur and oxygen atoms.
• A representation of electron-pair sharing that satisfies the octet rule for most atoms.
• Flexibility of sulfur to extend beyond the octet rule with double bonds.

In addition, the sulfite ion's negative charge influences its bonding characteristics with other particles. Positively charged ions, such as \( \mathrm{H}^+ \), are attracted to the negatively charged oxygens, showcasing the role of ionic interactions in determining chemical reactivity.