Problem 96 - Chemistry Principles and Practice

Question

The compound \(\mathrm{CF}_{3} \mathrm{SF}_{5}\) was recently identified in Earth's atmosphere and is a potential greenhouse gas. If the carbon atom and sulfur atom are bonded together, what is the Lewis structure of this compound? Do both central atoms follow the octet rule?

Step-by-Step Solution

Answer

The Lewis structure shows \( \mathrm{C} \)-\(\mathrm{S} \) centered with \( \mathrm{C} \) bonded to 3 \( \mathrm{F} \) and \( \mathrm{S} \) to 5 \( \mathrm{F} \); \( \mathrm{C} \) follows octet, \( \mathrm{S} \) doesn't.

Step 1: Identify the Central Atoms

In the molecule \( \mathrm{CF}_3\mathrm{SF}_5 \), identify the carbon (C) and sulfur (S) atoms as the central atoms. As specified, the carbon and sulfur atoms are bonded together.

Step 2: Count Valence Electrons

Each fluorine atom (\( \mathrm{F} \)) has 7 valence electrons, totaling 56 for 8 fluorines. Carbon (\( \mathrm{C} \)) has 4, and sulfur (\( \mathrm{S} \)) has 6. The total is \( 4 + 6 + 8 \times 7 = 70 \) valence electrons.

Step 3: Arrange Electrons Around Central Atoms

Draw a bond between \( \mathrm{C} \) and \( \mathrm{S} \). Attach three \( \mathrm{F} \) atoms to \( \mathrm{C} \) and five \( \mathrm{F} \) atoms to \( \mathrm{S} \). Each bond involves 2 electrons, and fluorines complete their octets with lone pairs.

Step 4: Check the Octet Rule

Each \( \mathrm{F} \) atom around \( \mathrm{C} \) and \( \mathrm{S} \) has 8 electrons (1 bond + 3 lone pairs), satisfying the octet rule. \( \mathrm{C} \) is bonded to \( \mathrm{S} \) and three \( \mathrm{F} \) atoms, having 8 electrons. \( \mathrm{S} \) is bonded to \( \mathrm{C} \) and five \( \mathrm{F} \) atoms (10 electrons), exceeding octet but allowed for \( \mathrm{S} \).

Step 5: Draw the Lewis Structure

Represent each bond as a line. Draw \( \mathrm{C} \) bonded to \( \mathrm{S} \) in the center. \( \mathrm{C} \) is connected to three \( \mathrm{F} \) atoms, while \( \mathrm{S} \) is connected to five \( \mathrm{F} \) atoms each by single bonds. Add pairs of dots around the fluorines for lone pairs.

Key Concepts

Octet Rule

The Octet Rule is a fundamental concept in chemistry that refers to the tendency of atoms to prefer having eight electrons in their valence shell. This configuration is similar to that of noble gases, which are known for their stability.
To better understand the Octet Rule, let's break it down:

• Atoms achieve octet either by sharing, losing, or gaining electrons through chemical bonds.
• This rule is particularly applicable for the second-period elements, such as carbon, nitrogen, and oxygen.
• While most common in these elements, the Octet Rule also helps explain bonding in many other compounds.

In the molecule \( \mathrm{CF}_3\mathrm{SF}_5 \), the carbon atom follows the Octet Rule by forming four bonds—three with fluorine and one with sulfur. Each of these bonds uses two electrons, thus completing carbon's octet. On the other hand, the sulfur atom is an exception to the Octet Rule. Sulfur, being in period 3, can have expanded valence shells and accommodate more than eight electrons, allowing it to bond with five fluorine atoms, resulting in what is called an "expanded octet." This ability allows sulfur in \( \mathrm{CF}_3\mathrm{SF}_5 \) to exceed the typical octet with ten electrons surrounding it.

Valence Electrons

Valence electrons are the electrons present in the outermost shell of an atom. These electrons are crucial because they determine how an atom interacts and bonds with other atoms. Understanding valence electrons can help predict the chemical properties and reactivity of an element.
In the context of \( \mathrm{CF}_3\mathrm{SF}_5 \), identifying the number of valence electrons is a vital step in drawing the Lewis structure:

• Fluorine (\(\mathrm{F}\)) has 7 valence electrons, contributing significantly to the total electron count.
• Carbon (\(\mathrm{C}\)) has 4 valence electrons, which allows it to form up to four bonds.
• Sulfur (\(\mathrm{S}\)) comes with 6 valence electrons, making it versatile in bonding scenarios.
• Totaling all these, we have 70 valence electrons to account for in \( \mathrm{CF}_3\mathrm{SF}_5 \).

This calculation involves simple arithmetic but provides the foundation for successfully arranging and bonding atoms in a structure. When constructing the Lewis structure, each bond accounts for two valence electrons, and balancing those with lone pairs achieves the full valence electron count.

Central Atom

In a molecular structure, the central atom is the one to which other atoms are bonded. Choosing the correct central atom is crucial for correctly constructing a molecule's Lewis structure.
For \( \mathrm{CF}_3\mathrm{SF}_5 \), both carbon and sulfur serve as central atoms. Here is why:

• The central atom is usually the atom with the lowest electronegativity (except hydrogen).
• Carbon, being less electronegative than fluorine, efficiently bonds in the molecule's center, allowing an optimal configuration.
• Sulfur, similar to carbon, can serve as a central atom due to its capacity for expanded octets, thus accommodating more bonds.
• Each central atom acts as a hub for bonding, with carbon connecting to three fluorines and sulfur to five.

Understanding which atoms to designate as central allows the creation of a stable and representative Lewis structure. This knowledge helps in predicting molecular geometry, polarities, and reactivity characteristics, all essential in further chemical understanding.