Question

Determine the molecular geometry about each interior atom and draw each molecule. (Skeletal structure is indicated in parentheses.) \begin{equation}\begin{array}{l}{\text { a. } C_{2} \mathrm{H}_{2}(\mathrm{HCCH})} \\ {\text { b. } \mathrm{C}_{2} \mathrm{H}_{4}\left(\mathrm{H}_{2} \mathrm{CCH}_{2}\right)} \\ {\text { c. } \mathrm{C}_{2} \mathrm{H}_{6}\left(\mathrm{H}_{3} \mathrm{CCH}_{3}\right)}\end{array} \end{equation}

Short answer

The molecular geometry for C2H2 is linear, for C2H4 is trigonal planar, and for C2H6 is tetrahedral. Each structure features the carbon atoms in different geometries due to the number of bonds and lone pairs around them.

Step-by-step solution

Determine Electron Pair Geometry

First, determine the total number of valence electrons for each interior atom. Then use the VSEPR theory to determine the electron pair geometry. For carbon (C), which is tetravalent, if there are four areas of electron density, the electron pair geometry is tetrahedral.

Identify the Molecular Geometry for C2H2

For C2H2 (HCCH), each carbon atom has two areas of electron density—formed by one triple bond and one single bond. According to VSEPR theory, this leads to a linear molecular geometry for each carbon atom.

Draw C2H2 Molecular Structure

Draw C2H2 with H-C≡C-H. The carbons are connected by a triple bond, and each carbon is bonded to one hydrogen, forming a linear structure.

Identify the Molecular Geometry for C2H4

For C2H4 (H2CCH2), each carbon atom has three areas of electron density—formed by one double bond and two single bonds. This results in a trigonal planar electron pair geometry and, considering the atoms present, a trigonal planar molecular geometry for each carbon atom.

Draw C2H4 Molecular Structure

Draw C2H4 as H2C=CH2. The carbons are connected by a double bond and each carbon is bonded to two hydrogen atoms, forming a trigonal planar structure.

Identify the Molecular Geometry for C2H6

For C2H6 (H3CCH3), each carbon atom has four areas of electron density—formed by one single bond with the other carbon atom and three single bonds with hydrogen atoms. This leads to a tetrahedral molecular geometry for each carbon atom.

Draw C2H6 Molecular Structure

Draw C2H6 with H3C-CH3. The two carbons are connected by a single bond, and each carbon is bonded to three hydrogen atoms, forming a tetrahedral structure.

Key concepts

Understanding VSEPR Theory

To begin with, let's delve into the VSEPR (Valence Shell Electron Pair Repulsion) theory. This model allows chemists to predict the geometry of a molecule by considering the repulsion between electron pairs around a central atom. The fundamental premise is that electron pairs will position themselves as far apart as possible to minimize repulsion.

VSEPR theory classifies electron pair geometry based on the number of bonding and non-bonding pairs around a central atom. For example, two electron domains form a linear geometry, while four electron domains could arrange themselves in a tetrahedral geometry. In our exercise, we evaluated electron pair geometries by first counting the valence electrons and areas of electron density around the central carbon atoms in molecules such as C2H2, C2H4, and C2H6.

It's important to remember that electron pair geometries are not always the same as molecular geometries. The former refers to the spatial arrangement of all electron pairs (bonding and non-bonding), while the latter describes the arrangement of only the atoms in a molecule. Understanding this distinction is critical when predicting molecular structure using VSEPR theory.

Decoding Valence Electrons

Valence electrons play a pivotal role in chemical bonding and molecular structure. They are the electrons located in the outermost shell of an atom and are involved in forming bonds with other atoms. The number of valence electrons an element has can often be deduced from its group number on the periodic table.

For instance, as in our exercise, carbon (C) has four valence electrons since it belongs to group 14. These are used to form bonds with other atoms such as hydrogen (H). Hydrogen, located in group 1, has one valence electron and requires one more to complete its outer shell. When analyzing molecules in VSEPR theory, we first count the valence electrons to understand how an atom can bond and then predict the electron pair and molecular geometries.

• For C2H2, the carbon has four valence electrons, two of which are involved in the triple bond and two in the single bond.
• For C2H4, each carbon atom also uses all four valence electrons, forming a double bond with another carbon and two single bonds with hydrogen atoms.
• In C2H6, every carbon uses its four valence electrons to bond with one another and with three hydrogen atoms, resulting in a total of four single bonds.

Through practice and understanding the behaviors of valence electrons, students can enhance their predictive abilities concerning molecular configurations.

Electron Pair Geometry Essentials

With the VSEPR theory and valence electrons in mind, we move to electron pair geometry, which explains the three-dimensional arrangement of all electron pairs around a central atom, including both bonding and lone pairs.

In the case of our exercise with hydrocarbons C2H2, C2H4, and C2H6, the electron pair geometry was used as a stepping stone to derive the molecular geometry. Here's a concise breakdown:

• C2H2 has a linear electron pair geometry because each carbon is surrounded by two electron pairs involved in bonding.
• C2H4 has a trigonal planar electron pair geometry, with each carbon atom surrounded by three bonding pairs of electrons.
• Lastly, for C2H6, the tetrahedral electron pair geometry comes into play; each carbon has four bonding electron pairs.

In summary, understanding electron pair geometry not only helps in visualizing the shape of the entire electron cloud around a central atom but also aids in predicting the physical and chemical properties of molecules. Students will benefit from grasping that electron pair geometries—linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral—lay the foundation for understanding complex molecular architectures.