Question

For each of the following molecules, indicate the bond angle expected between the central atom and any two adjacent chlorine atoms. a. \(\mathrm{Cl}_{2} \mathrm{O}\) b. \(\mathrm{CCl}_{4}\) c. \(\mathrm{BeCl}_{2}\) d. \(\mathrm{BCl}_{3}\)

Short answer

The bond angles between the central atom and any two adjacent chlorine atoms are as follows: a. For \(\mathrm{Cl}_{2} \mathrm{O}\), the bond angle (Cl-O-Cl) is approximately 109.5°. b. For \(\mathrm{CCl}_{4}\), the bond angle (Cl-C-Cl) is approximately 109.5°. c. For \(\mathrm{BeCl}_{2}\), the bond angle (Cl-Be-Cl) is 180°. d. For \(\mathrm{BCl}_{3}\), the bond angle (Cl-B-Cl) is 120°.

Step-by-step solution

a. \(\mathrm{Cl}_{2} \mathrm{O}\)

First we need to identify the central atom in \(\mathrm{Cl}_{2}\mathrm{O}\), which is oxygen (O). There are two chlorine (Cl) atoms bonded to the central atom, and oxygen has a total of 2 lone pairs. According to the VSEPR theory, the shape of the molecule is bent, and the oxygen atom has a 2 electron pair geometry. Therefore, the predicted bond angle for Cl-O-Cl is approximately 109.5°.

b. \(\mathrm{CCl}_{4}\)

For \(\mathrm{CCl}_{4}\), the central atom is carbon (C). There are four chlorine (Cl) atoms bonded to the central carbon atom and no lone pairs on the carbon atom. According to VSEPR theory, this molecule has a tetrahedral geometry. Therefore, the predicted bond angle for Cl-C-Cl is approximately 109.5°.

c. \(\mathrm{BeCl}_{2}\)

In the case of \(\mathrm{BeCl}_{2}\), the central atom is beryllium (Be). There are two chlorine (Cl) atoms bonded to the central beryllium atom and no lone pairs on the central atom. According to the VSEPR theory, the molecule has a linear geometry. Therefore, the predicted bond angle for Cl-Be-Cl is 180°.

d. \(\mathrm{BCl}_{3}\)

Finally, for \(\mathrm{BCl}_{3}\), the central atom is boron (B). There are three chlorine (Cl) atoms bonded to the central boron atom and no lone pairs on the central atom. According to VSEPR theory, this molecule has a trigonal planar geometry. Therefore, the predicted bond angle for Cl-B-Cl is 120°.

Key concepts

Molecular Geometry

Molecular geometry is all about the three-dimensional arrangement of atoms within a molecule. It helps us understand how atoms are positioned relative to each other and predicts the shape of the molecule.
The shape of a molecule can greatly affect its properties, such as reactivity, polarity, and even color. According to the VSEPR (Valence Shell Electron Pair Repulsion) theory, electron pairs around a central atom will arrange themselves to be as far apart as possible to minimize repulsion.
This leads to specific geometric arrangements:

• Bent geometry, like in Cl₂O, occurs when there are two bonds and lone pairs causing a non-linear arrangement.
• Tetrahedral geometry, seen in CCl₄, where four bonds are equally spaced, forming a three-dimensional shape.
• Linear geometry, found in BeCl₂, with its two bonds aligned straight through the central atom.
• Trigonal planar geometry, as in BCl₃, where three bonds form a flat plane around the central atom.

Understanding these shapes helps predict how molecules behave in chemical reactions.

Bond Angles

Bond angles refer to the angles between adjacent chemical bonds at an atom in a molecule. These are determined by the molecular geometry and directly influence the molecule's shape.
In Cl₂O, the Cl-O-Cl bond angle is slightly less than 109.5°, which is typical for a bent shape caused by the lone pairs on the oxygen.
For CCl₄, the Cl-C-Cl bond angle is exactly 109.5°, matching the ideal tetrahedral angle due to its symmetrical arrangement.
In BeCl₂, the shape is linear, so the bond angle between Cl-Be-Cl is 180°, perfectly straight.
Lastly, in BCl₃, the angle between Cl-B-Cl is 120°, characteristic of a flat trigonal planar geometry.
These angles are essential for predicting how molecules interact with each other and with other compounds.

Central Atom

The concept of a central atom is pivotal in determining a molecule's structure because this atom usually bonds to multiple atoms.
In our examples:

• Oxygen (O) acts as the central atom in Cl₂O, forming bonds with two chlorine atoms while maintaining two lone pairs, influencing the molecule's bent shape.
• Carbon (C) serves as the central atom in CCl₄, which connects to four chlorine atoms, creating a tetrahedral geometry.
• Beryllium (Be) is the central atom in BeCl₂, engaging with two chlorine atoms, leading to a linear arrangement.
• Boron (B) functions as the central atom in BCl₃, linking with three chlorine atoms and establishing a trigonal planar geometry.

Understanding which atom is central helps predict the different geometries and bonding patterns in molecules.

Chemical Bonding

Chemical bonding refers to the forces holding atoms together in a molecule. Bonds form when atoms share or transfer electrons, creating a variety of molecular structures.
In covalent bonds, such as in Cl₂O, CCl₄, BeCl₂, and BCl₃, atoms share electrons to achieve stability.

• In Cl₂O, the sharing of electrons between oxygen and chlorine creates a stable, bent molecular form due to lone pairs on oxygen.
• In CCl₄, carbon forms stable tetrahedral bonds with four chlorine atoms, each sharing a pair of electrons with carbon.
• BeCl₂ features beryllium sharing electrons with two chlorine atoms, resulting in linear bonds.
• BCl₃ holds boron centrally, sharing electrons with three chlorine atoms in a planar structure.

Understanding these bonding interactions is fundamental to exploring how molecules are structured and react chemically.