Question

Gaseous aluminum chloride exists as a dimer, \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\), with two chlorines bridging between the two aluminum atoms: \(\mathrm{Cl}_{2} \mathrm{AlCl}_{2} \mathrm{AlCl}_{2}\). Draw a Lewis structure for \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\) and for \(\mathrm{AlCl}_{3}\). Discuss a reason why \(\mathrm{AlCl}_{3}\) forms \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\).

Short answer

In the Lewis structure of \(\mathrm{AlCl}_{3}\), Al shares its 3 electrons with 3 Cl atoms to form three covalent bonds. Each Cl atom also has three lone pairs of electrons. For \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\), it forms a dimer with four Cl atoms bonded to each Al atom – two through direct bonds and two through bridging bonds shared by two Al atoms, hence achieving the octet configuration. Dimerization makes gaseous aluminum chloride more stable.

Step-by-step solution

Lewis Structure for \(\mathrm{AlCl}_{3}\)

Aluminum (Al) has 3 valence electrons, and Chlorine (Cl) has 7. In \(\mathrm{AlCl}_{3}\), the aluminum atom shares its 3 electrons with three chlorine atoms to form three covalent bonds, reaching an octet configuration in the process. Each chlorine atom also contributes an electron to the bond formation, reaching an octet configuration as well. The structure can be drawn with aluminum in the center connected to the three chlorine atoms through single bonds. Each chlorine atom will also have three pairs of electrons that are not involved in bonding.

Lewis Structure for \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\)

In the case of \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\), it forms a dimer with two chlorines bridging between the aluminum atoms. Hence, the aluminum atoms are bonded to four chlorine atoms, emitting two lone pairs of electrons on each bridging chlorine atom. Structurally, the two bridging chlorine atoms sit between the two aluminum atoms, with each aluminum atom also connected to two other chlorine atoms.

Discussion on Dimerization of \(\mathrm{AlCl}_{3}\) into \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\)

\(\mathrm{AlCl}_{3}\) forms \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\) due to the absence of an octet configuration with \(\mathrm{AlCl}_{3}\). Aluminum only has 6 electrons around it after forming \(\mathrm{AlCl}_{3}\). Through dimerization, each aluminum atom can share one more pair of electrons from a chlorine atom, hence achieving the octet configuration. The dimerization thus results in more stable structures for gaseous aluminum chloride.

Key concepts

Aluminum Chloride Dimerization

Aluminum chloride, \(\text{AlCl}_3\), is a fascinating compound because under certain conditions it can form something called a dimer. A dimer means two molecules are connected together. In this case, it forms \(\text{Al}_2\text{Cl}_6\). This occurs because \(\text{AlCl}_3\) is not stable in its lone form. In the gaseous state, two \(\text{AlCl}_3\) units join to make a single \(\text{Al}_2\text{Cl}_6\) molecule.

• The dimerization process involves two chlorine atoms acting as bridges between two aluminum atoms.
• The structural change allows each aluminum atom to reach a more stable electron configuration replicating full outer shells by sharing electrons.

This sharing of electrons makes the compound \(\text{Al}_2\text{Cl}_6\) more stable than having separate \(\text{AlCl}_3\) molecules in environments like the gaseous phase.

Valence Electrons

Valence electrons are the outermost electrons in an atom and are essential in determining how atoms will bond with each other. Aluminum (Al) has three valence electrons, while chlorine (Cl) has seven. In the case of aluminum chloride:

• Aluminum needs to bond in a way that utilizes its three valence electrons to achieve stability.
• Each chlorine atom, with its seven valence electrons, seeks one more electron to complete its octet.

The interaction between the valence electrons of aluminum and chlorine is what leads to the formation of covalent bonds in \(\text{AlCl}_3\) and eventually the creation of the \(\text{Al}_2\text{Cl}_6\) dimer. Understanding this electron exchange is key to grasping why molecules form the way they do.

Covalent Bonds

Covalent bonds are formed when two atoms share electron pairs. In \(\text{AlCl}_3\), aluminum shares its three valence electrons with chlorine atoms to form three covalent bonds.

• Each shared pair of electrons constitutes a single covalent bond.
• All chlorine atoms reach a stable octet configuration by sharing one electron with aluminum.

By comparing \(\text{AlCl}_3\) to the dimer \(\text{Al}_2\text{Cl}_6\), it's easy to see how sharing electrons helps atoms complete their outer shells. Bridging chlorines participate in additional covalent bonding between aluminum atoms, helping aluminum achieve a full octet, leading to stable structures in gaseous aluminum chloride. Covalent bonds thus play a critical role in stabilizing both the \(\text{AlCl}_3\) molecule and its dimerized form.

Octet Configuration

The octet rule is a cornerstone in chemistry, suggesting that atoms tend to combine to complete their outer electron shells, usually with eight electrons. For most main-group elements, especially in Groups 1A to 7A of the periodic table:

• This rule helps predict how atoms will bind with each other.
• Atoms achieve extra stability by having a configuration similar to noble gases.

In the case of \(\text{AlCl}_3\), aluminum initially does not achieve an octet configuration with its three electrons available for bonding. It only gets six. By forming \(\text{Al}_2\text{Cl}_6\), aluminum can share additional electrons from the bridging chlorines, enabling each aluminum atom to reach the desired octet. Understanding the octet rule explains why \(\text{AlCl}_3\) forms a dimer - to stabilize electronically by satisfying this rule through mutual sharing of electrons.