Question

For the indicated atom in each of the following molecules, give the number and the arrangement of the electron pairs around that atom. a. \(\mathrm{C}\) in \(\mathrm{CCl}_{4}\) b. Ge in \(\mathrm{GeH}_{4}\) c. \(\mathrm{B}\) in \(\mathrm{BF}_{3}\)

Short answer

a. For \(\mathrm{C}\) in \(\mathrm{CCl}_{4}\): Tetrahedral arrangement with 4 electron pairs. b. For Ge in \(\mathrm{GeH}_{4}\): Tetrahedral arrangement with 4 electron pairs. c. For \(\mathrm{B}\) in \(\mathrm{BF}_{3}\): Trigonal planar arrangement with 3 electron pairs.

Step-by-step solution

Determine the valence electrons around the central atom

First, we need to count the valence electrons around the central atom of each molecule. a. For \(\mathrm{C}\) in \(\mathrm{CCl}_{4}\): Carbon (C) has 4 valence electrons and each Chlorine (Cl) has 7. Therefore, the total number of valence electrons around Carbon is 4 + 4(7) = 32. b. For Ge in \(\mathrm{GeH}_{4}\): Germanium (Ge) has 4 valence electrons and each Hydrogen (H) has 1. Therefore, the total number of valence electrons around Germanium is 4 + 4(1) = 8. c. For \(\mathrm{B}\) in \(\mathrm{BF}_{3}\): Boron (B) has 3 valence electrons and each Fluorine (F) has 7. Therefore, the total number of valence electrons around Boron is 3 + 3(7) = 24.

Determine the electron domain geometry

Now, we need to find the electron domain geometry using the number of valence electrons found in step 1. a. In \(\mathrm{CCl}_{4}\), the central Carbon atom has 4 electron domains (1 from each bond to Chlorine), which gives it a tetrahedral electron domain geometry. b. In \(\mathrm{GeH}_{4}\), the central Germanium atom has 4 electron domains (1 from each bond to Hydrogen), which also gives it a tetrahedral electron domain geometry. c. In \(\mathrm{BF}_{3}\), the central Boron atom has 3 electron domains (1 from each bond to Fluorine), which gives it a trigonal planar electron domain geometry.

Determine the arrangement and number of electron pairs

Finally, we can determine the arrangement and number of electron pairs around the central atoms in each molecule by considering the electron domain geometry found in step 2. a. For \(\mathrm{C}\) in \(\mathrm{CCl}_{4}\): Since the electron domain geometry is tetrahedral, the arrangement of electron pairs around one Carbon atom is also tetrahedral, with 4 electron pairs. b. For Ge in \(\mathrm{GeH}_{4}\): Since the electron domain geometry is tetrahedral, the arrangement of electron pairs around the Germanium atom is also tetrahedral, with 4 electron pairs. c. For \(\mathrm{B}\) in \(\mathrm{BF}_{3}\): Since the electron domain geometry is trigonal planar, the arrangement of electron pairs around the Boron atom is trigonal planar, with 3 electron pairs.

Key concepts

Valence Electrons

Valence electrons are crucial for understanding the structure and reactivity of molecules. These are the electrons found in the outermost shell of an atom and are responsible for forming chemical bonds.
They help determine the atom's ability to bond with other atoms.
Here are a few key points about valence electrons:

• They are involved in forming covalent bonds by sharing between atoms or ionic bonds through the transfer of electrons.
• Valence electrons play a significant role in determining the molecule's geometry since they dictate how atoms align themselves in a molecule.
• The number of valence electrons influences the possible electron domains which are areas of electron density around the central atom.

In molecules like CCl extsubscript{4}, each chlorine atom uses a part of its seven valence electrons to form bonds with carbon, which has four valence electrons.
This results in a stable electronic configuration where the outer electron shell is filled. In contrast, GeH extsubscript{4} follows a similar process with germanium sharing its four valence electrons with hydrogen atoms.
In BF extsubscript{3}, boron shares its three valence electrons with fluorine, facilitating the formation of bonds that ultimately determine the geometry of the molecule.

Tetrahedral Geometry

Tetrahedral geometry refers to the specific arrangement of atoms around a central atom in a molecule where four bonds create a three-dimensional shape resembling a pyramid with a triangular base.
This is a common geometry for molecules where the central atom has four electron domains.

• Each bond angle in a perfect tetrahedron is around 109.5°.
• The molecules CCl extsubscript{4} and GeH extsubscript{4} exhibit tetrahedral geometry.
• This geometry optimizes the distance between electron domains, minimizing repulsion and creating a stable molecular structure.

The four electron domains in CCl extsubscript{4} result from the bond formation between carbon and each chlorine atom.
Similarly, in GeH extsubscript{4}, germanium bonds with hydrogen, adopting the same geometry.
This shape ensures that the electron pairs, whether used in bonding or as lone pairs, adjust themselves in a way that they are as far apart from each other as possible, which is a fundamental principle explained by the VSEPR (Valence Shell Electron Pair Repulsion) theory.

Trigonal Planar Geometry

Trigonal planar geometry occurs when three groups of electrons surround a central atom, arranging themselves at 120° angles on a single plane.
This arrangement is often observed in molecules where the central atom forms three bonds without needing a full octet.
Here’s what to know about trigonal planar geometry:

• The central atom has three electron domains, all of which are bonding domains.
• Each bond angle in a perfect trigonal planar geometry is precisely 120°.
• Boron trifluoride ( BF extsubscript{3}) showcases this geometry effectively.

In BF extsubscript{3}, the central boron atom’s three electron domains come from its bonds with three fluorine atoms.
Rather than adopting a shape where electron pairs are as far apart three-dimensionally as possible, BF extsubscript{3} arranges these bonds in a flat, triangular shape.
Due to boron's unique property of being content without an octet, it forms stable molecules like BF extsubscript{3}, featuring this geometry without lone pairs causing distortion.