Problem 78 - Chemistry The Molecular Science

Question

Lapis lazuli is an aluminum silicate whose brilliant blue color is due to the presence of \(\mathrm{S}_{3}^{-}\) ions. Write a plausible Lewis structure for this ion.

Step-by-Step Solution

Answer

The plausible Lewis structure involves S-S-S bonds with delocalized electrons, showing resonance between different double bond placements.

Step 1: Determine the Total Number of Valence Electrons

The sulfide ion, \(\mathrm{S}_3^-\), consists of three sulfur atoms. Each sulfur atom has six valence electrons. Since there is a \(-1\) charge overall, we need to add one more electron. Thus, the total number of valence electrons is: \(3 \times 6 + 1 = 19\) electrons.

Step 2: Draw a Basic Skeleton Structure

Place the three sulfur atoms in a linear arrangement, and use a single bond (two electrons) to connect each pair of adjacent sulfur atoms: \(\mathrm{S} - \mathrm{S} - \mathrm{S}\). Each bond uses 2 electrons, so this uses a total of 4 electrons.

Step 3: Distribute Remaining Electrons as Lone Pairs

After subtracting the 4 electrons used for bonds from the total of 19, we have 15 electrons left. Distribute these electrons as lone pairs around the sulfur atoms to satisfy the octet rule as much as possible for each atom. The terminal sulfurs each get 6 electrons to complete their octets, and the central sulfur gets 3 electrons.

Step 4: Adjust for Charge

The central sulfur atom contains an odd number of electrons, which is resolved by assigning one extra nonbonding electron to this atom. As a result, the central sulfur carries the additional electron suggested by the \(-1\) charge. This structure may involve resonance between different possibilities of single and double bond distributions.

Step 5: Evaluate Resonance Structures

The structure you've developed should show resonance because sulfur atoms can form bonds with varying orders. Possible forms include a resonance where two possible double bonds exist between pairs of sulfur atoms. Draw resonance structures by delocalizing the electrons to show different double bond locations.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial for bonding. They determine how atoms interact and form molecules.
Understanding the number of valence electrons helps us draw the Lewis structure of an ion or molecule.
- For the sulfide ion, \( \mathrm{S}_3^- \), each sulfur atom brings 6 valence electrons.- Since there's a negative charge, we add one more electron for the ion's overall charge.Thus, the sulfide ion has a total of 19 valence electrons. Calculating these correctly is the first step toward drawing an accurate Lewis structure.
This concept allows us to visualize how atoms share electrons to form stable compounds.

Resonance Structures

Resonance structures are multiple ways to draw a molecule's Lewis structure.- These structures represent the same molecule but differ in the placement of electrons.- Resonance is important when a single Lewis structure cannot accurately depict a molecule.For \( \mathrm{S}_3^- \), resonance involves different arrangements of double bonds between sulfur atoms. The electrons can move between the bonds, depicting a blend rather than fixed positions. This delocalization stabilizes the molecule since the electrons are spread over more than one position.
Visualizing resonance structures helps us understand the flexibility in electron distribution in a molecule.

Lone Pairs

Lone pairs are pairs of electrons on an atom that are not involved in bonding.- They are crucial in determining the shape and reactivity of a molecule.- In the \( \mathrm{S}_3^- \) ion, after accounting for bonds, the remaining valence electrons are distributed as lone pairs.Each terminal sulfur in this ion receives pairs of electrons to complete their octets, while the central sulfur gains an odd electron. This leads to variations in formal charges but is necessary to satisfy the octet rule as much as possible. Lone pairs influence molecular geometry and can affect how molecules interact with one another.