Problem 13 - Chemistry The Molecular Science

Question

Draw the Lewis structure and identify the molecular shape of each molecule. (a) \(\mathrm{BeH}_{2}\) (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{BH}_{3}\) (d) \(\mathrm{SeCl}_{6}\) (e) \(\mathrm{PF}_{3}\)

Step-by-Step Solution

Answer

(a) Linear, (b) Tetrahedral, (c) Trigonal planar, (d) Octahedral, (e) Trigonal pyramidal.

Step 1: Draw Lewis Structure for \(\mathrm{BeH}_{2}\)

Beryllium \(\mathrm{Be}\) has 2 valence electrons and each hydrogen \(\mathrm{H}\) has 1 valence electron. Beryllium shares its electrons with the two hydrogens, forming two single bonds. The Lewis structure is:\[\mathrm{H}-\mathrm{Be}-\mathrm{H}\]

Step 2: Determine Molecular Shape for \(\mathrm{BeH}_{2}\)

With 2 bond pairs and 0 lone pairs around beryllium, \(\mathrm{BeH}_{2}\) adopts a linear shape according to VSEPR theory.

Step 3: Draw Lewis Structure for \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)

Carbon \(\mathrm{C}\) has 4 valence electrons, each hydrogen \(\mathrm{H}\) has 1, and each chlorine \(\mathrm{Cl}\) has 7. Carbon forms two single bonds with hydrogen and two single bonds with chlorine. The Lewis structure is:\[\mathrm{H}-\mathrm{C}-\mathrm{H}\]\[\mid\hspace{1cm}\mid\]\[\mathrm{Cl}\hspace{0.7cm}\mathrm{Cl}\]

Step 4: Determine Molecular Shape for \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)

With 4 regions of electron density (all bonded) around carbon, \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) has a tetrahedral shape.

Step 5: Draw Lewis Structure for \(\mathrm{BH}_{3}\)

Boron \(\mathrm{B}\) has 3 valence electrons and bonds with three hydrogens. Each hydrogen contributes 1 valence electron. The Lewis structure is:\[\begin{array}{c} \quad \mathrm{H} \ \mid \ \mathrm{B}-\mathrm{H}\ \mid \ \quad \mathrm{H} \end{array}\]

Step 6: Determine Molecular Shape for \(\mathrm{BH}_{3}\)

With 3 bond pairs and 0 lone pairs around boron, \(\mathrm{BH}_{3}\) has a trigonal planar shape.

Step 7: Draw Lewis Structure for \(\mathrm{SeCl}_{6}\)

Selenium \(\mathrm{Se}\) has 6 valence electrons, and each chlorine \(\mathrm{Cl}\) has 7. Selenium shares electrons with six chlorines, forming six single bonds. The Lewis structure is an octahedral arrangement of \(\mathrm{Cl}\) atoms around \(\mathrm{Se}\).

Step 8: Determine Molecular Shape for \(\mathrm{SeCl}_{6}\)

With 6 bond pairs and 0 lone pairs around selenium, \(\mathrm{SeCl}_{6}\) has an octahedral shape.

Step 9: Draw Lewis Structure for \(\mathrm{PF}_{3}\)

Phosphorus \(\mathrm{P}\) has 5 valence electrons and shares electrons with three fluorines \(\mathrm{F}\), each having 7 valence electrons. The Lewis structure includes a lone pair on phosphorus:\[\begin{array}{c} \quad \mathrm{F} \ \mid \ \mathrm{P}-\mathrm{F}\ \mid \ \quad \mathrm{F} \end{array}\]

Step 10: Determine Molecular Shape for \(\mathrm{PF}_{3}\)

With 3 bond pairs and 1 lone pair around phosphorus, \(\mathrm{PF}_{3}\) has a trigonal pyramidal shape.

Key Concepts

VSEPR Theory

The VSEPR Theory, which stands for Valence Shell Electron Pair Repulsion, is a fundamental concept in understanding the three-dimensional shapes of molecules. It is based on the idea that electron pairs surrounding a central atom will repel each other and arrange themselves as far apart as possible. This helps minimize repulsion and determine the molecular shape. VSEPR Theory helps predict the geometry of a molecule by considering the number of electron pairs, both bonded and non-bonded, around a central atom.
For instance, in the molecule \(\text{BeH}_2\), the beryllium atom has two bond pairs and no lone pairs. According to VSEPR Theory, the electron pairs will arrange themselves linearly to minimize repulsion. As a result, \(\text{BeH}_2\) has a linear shape. Understanding VSEPR helps visualize and predict molecular geometries, which is crucial in the study of chemistry.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. It dictates the shape and structure that the molecule assumes due to the spatial distribution of its constituent atoms. The shape of a molecule affects its physical and chemical properties, including polarity and reactivity.
Examining the structure of \(\text{CH}_2 \text{Cl}_2\), we find that it adopts a tetrahedral geometry. This occurs due to four regions of electron density surrounding the central carbon atom, all of which are bonded pairs. The angles between these bonds are approximately \(109.5^\circ\), creating a three-dimensional shape akin to a pyramid with a triangular base. Recognizing molecular geometry aids in predicting the behavior and interaction of molecules in different chemical reactions.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in chemical bonding. These are the electrons involved when atoms form bonds with one another, and they determine an atom's ability to bond and form molecules. The number of valence electrons varies across different elements in the periodic table but is essential for understanding bond formation.
Let's consider the molecule \(\text{SeCl}_6\). Selenium, the central atom, has six valence electrons, and each chlorine has seven. By sharing its valence electrons with the chlorines through single bonds, all atoms achieve a complete electron configuration. The shared electrons fill the valence shell of selenium, leading to the octahedral shape of \(\text{SeCl}_6\). Knowing the number of valence electrons helps in constructing Lewis structures and predicting molecular shapes.

Chemical Bonding

Chemical bonding refers to the force that holds atoms together in molecules. There are several types of chemical bonds, including ionic, covalent, and metallic bonds, depending on how electrons are shared or transferred between atoms.
A covalent bond, like those in \(\text{PF}_3\), involves the sharing of electron pairs between atoms. Phosphorus forms covalent bonds with three fluorines, each sharing one electron with phosphorus. Due to its three bonding pairs and one lone pair, \(\text{PF}_3\) exhibits a trigonal pyramidal shape. Chemical bonding is essential for the stability and formation of compounds, influencing their structural and chemical characteristics. Understanding chemical bonding is key to grasp both simple and complex chemical processes in molecular chemistry.