Chapter 10: Problem 82

Aluminum trichloride $\left(\mathrm{AlCl}_{3}\right)$ is an electron-deficient molecule. It has a tendency to form a dimer (a molecule made of two $\mathrm{AlCl}_{3}$ units): $$ \mathrm{AlCl}_{3}+\mathrm{AlCl}_{3} \longrightarrow \mathrm{Al}_{2} \mathrm{Cl}_{6} $$ (a) Draw a Lewis structure for the dimer. (b) Describe the hybridization state of $\mathrm{Al}$ in $\mathrm{AlCl}_{3}$ and $\mathrm{Al}_{2} \mathrm{Cl}_{6}$. (c) Sketch the geometry of the dimer. (d) Do these molecules possess a dipole moment?

Short Answer

Expert verified

$\mathrm{AlCl}_{3}$ is sp2 hybridized and has a trigonal planar structure, while $\mathrm{Al}_{2} \mathrm{Cl}_{6}$ is sp3 hybridized and has a tetrahedral structure. Both molecules do not have a dipole moment.

Step by step solution

Drawing Lewis Structure

A Lewis structure can be drawn for $\mathrm{Al}_{2} \mathrm{Cl}_{6}$ by linking each $\mathrm{Al}$ atom to three $\mathrm{Cl}$ atoms, satisfying the octet rule for the $\mathrm{Cl}$ atoms. In addition, a lone pair of electrons is also present on one $\mathrm{Al}$ and another $\mathrm{Al}$ from another $\mathrm{AlCl}_{3}$. These lone pairs from two separate $\mathrm{AlCl}_{3}$ are used to link them together to form the dimer $\mathrm{Al}_{2} \mathrm{Cl}_{6}$.

Describing the Hybridization State

In $\mathrm{AlCl}_{3}$, each $\mathrm{Al}$ atom is sp2 hybridized, which means it has a trigonal planar geometry around the $\mathrm{Al}$ atom. On the other hand, in $\mathrm{Al}_{2} \mathrm{Cl}_{6}$, $\mathrm{Al}$ is sp3 hybridized which means it follows a tetrahedral geometry around the $\mathrm{Al}$ atom.

Sketching the Geometry of the Dimer

The geometry of the molecule $\mathrm{Al}_{2} \mathrm{Cl}_{6}$ is tetrahedral around $\mathrm{Al}$ atom due to sp3 hybridization. Each $\mathrm{Al}$ atom is linked to three $\mathrm{Cl}$ atoms and one $\mathrm{Al}$ atom.

Determining Dipole Moment

In $\mathrm{Al}_{2} \mathrm{Cl}_{6}$, the $\mathrm{Cl}$ atoms around each $\mathrm{Al}$ are perfectly symmetrical. This means there is no net polarity, thus, the molecule does not possess a dipole moment. For $\mathrm{AlCl}_{3}$, it also does not possess a dipole moment because it has a trigonal planar structure, which makes the polarities to cancel out each other.

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Short Answer

Step by step solution

Drawing Lewis Structure

Describing the Hybridization State

Sketching the Geometry of the Dimer

Determining Dipole Moment

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