Problem 79 - 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 \(\mathrm{H}^{+}\). Does \(\mathrm{H}^{+}\) attach itself to the S atom or the O atom of \(\mathrm{SO}_{3}^{2-} ?\)

Step-by-Step Solution

Answer

The \(\mathrm{H}^+\) ion is likely to attach to an oxygen atom due to its lone pairs.

Step 1: Count Total Valence Electrons

The sulfite ion, \(\mathrm{SO}_3^{2-}\), consists of one sulfur atom and three oxygen atoms with an overall charge of \(-2\). Sulfur has 6 valence electrons, and each oxygen has 6 valence electrons. The extra \(-2\) charge adds 2 more electrons to the total count. Thus, the total number of valence electrons is \(6 + (3\times6) + 2 = 26\) electrons.

Step 2: Determine the Central Atom

The sulfur atom is less electronegative compared to oxygen and can form more bonds, making it a suitable central atom. Arrange sulfur in the center with the three oxygen atoms surrounding it.

Step 3: Create Bonds Between Atoms

Connect each oxygen atom to the sulfur atom with a single bond initially. This setup uses \(3\times2=6\) electrons, leaving us with \(26-6=20\) electrons to distribute.

Step 4: Distribute Remaining Electrons

Distribute the remaining electrons to fulfill the octet rule for each oxygen atom. Each oxygen needs 8 electrons, 2 of which are from the bond, so add 6 more electrons as lone pairs to each oxygen. Now, 3 oxygen atoms use \(3\times6=18\) electrons, leaving \(20-18=2\) electrons remaining.

Step 5: Assign Remaining Electrons to Central Atom

Place the remaining 2 electrons as a lone pair on the sulfur atom. Now all atoms follow the octet rule, with sulfur having an expanded octet (12 electrons, which is acceptable for sulfur).

Step 6: Evaluate the Interaction with \(\mathrm{H}^+\)

The \(\mathrm{H}^+\) ion, being a proton, often binds to atoms with lone pairs of electrons. In \(\mathrm{SO}_3^{2-}\), the oxygen atoms have lone pairs, whereas the sulfur does not, making an oxygen atom more likely to be the binding site for \(\mathrm{H}^+\).

Key Concepts

Valence Electrons

Valence electrons are crucial when understanding chemical bonds and molecular structures. These electrons are found in the outermost shell of an atom and participate in forming bonds.
For any molecule or ion, counting these electrons accurately provides insight into how atoms interact. For the sulfite ion ( SO_3^{2-} ), this involves adding up electrons from each atom as well as considering the overall charge.

• Sulfur has 6 valence electrons.
• Each oxygen atom also contributes 6 electrons.
• The -2 charge means we add 2 extra electrons.

Together, this sums up to 26 valence electrons needed to construct the Lewis structure. Understanding these electrons is the first step in drawing the electron dot structure and predicting bonding and structure.

Sulfite Ion

The sulfite ion, SO_3^{2-} , is an important species in chemistry, having both structural and reactive properties. It comprises one sulfur atom centrally surrounded by three oxygen atoms.
Notably, the overall -2 charge of the ion signifies it has gained two additional electrons. This reflects its ability to interact with other ions or molecules.
A key characteristic of the sulfite ion is its resonance; it exists in multiple forms where the lone pairs and double bonds can "move" among the oxygen atoms. This phenomenon gives it additional stability and allows it to participate in diverse chemical reactions.
Understanding the structure and charge distribution in sulfite ions is essential for predicting how they interact in solution.

Octet Rule

The octet rule is a guiding principle in chemistry that suggests atoms are most stable when they have eight electrons in their valence shell. This rule helps predict and explain bonding patterns, especially in neutral molecules and ions like sulfite.
The sulfite ion ( SO_3^{2-} ) must adhere to this rule while accommodating its 26 valence electrons.

• Each oxygen atom should have 8 electrons, including from bonds with sulfur, to satisfy the octet rule.
• Oxygen uses lone pairs to complete its octet.
• Sulfur can exceed the octet due to higher energy level d orbitals allowing additional electrons.

Achieving an octet provides stability. In molecules like sulfite, seeing how each atom achieves this arrangement helps to predict its reactivity and interaction with other species.

Central Atom

Selecting the central atom is a fundamental step in constructing a Lewis structure. This atom usually forms the most bonds and is often less electronegative. In the case of the sulfite ion ( SO_3^{2-} ), sulfur is chosen as the central atom.
Sulfur's ability to form multiple bonds suits it well for this position, and its lower electronegativity compared to oxygen makes it more willing to share electrons.

• Sulfur forms single bonds with each of the three surrounding oxygen atoms.
• The position and bonding pattern of sulfur allow efficient use of valence electrons.
• The choice of central atom impacts overall geometry and electronic distribution.

Considering these factors ensures a correct and balanced Lewis structure, paving the way to understanding the molecule's geometry and reactions it might undergo.