Question

Aluminium (llI) chloride forms dimer because (1) higher co-ordination number can be achicved. (2) aluminium has high ionization encrgy. (3) aluminium belongs to III group. (4) it cannot form a trimer.

Short answer

Option (1): Higher coordination number can be achieved.

Step-by-step solution

Understand the Problem

We need to determine the reason Aluminium (III) chloride forms a dimer.

Analyze the Options

Let's evaluate why each option might or might not be correct.

Option 1: Higher Coordination Number

Forming a dimer can indeed allow aluminum to increase its coordination number from 3 to 4 by bridging two chloride ions between two aluminum ions.

Option 2: High Ionization Energy

High ionization energy doesn't directly influence the formation of a dimer as it pertains to the energy required to remove an electron.

Option 3: Group III Element

Belonging to Group III doesn't exclusively explain why it would form a dimer instead of just being a characteristic of the element.

Option 4: Cannot Form a Trimer

The inability to form a trimer is not the reason for dimer formation. The formation of dimers is more about achieving stability and increasing coordination.

Conclusion

The primary reason Aluminium (III) chloride forms a dimer is to achieve a higher coordination number, making option (1) correct.

Key concepts

coordination number

Coordination number refers to the number of atoms or ions directly bonded to a central atom. In the case of Aluminium (III) chloride, each aluminum ion has a coordination number of three when it is not in a dimer. However, forming a dimer allows aluminum to increase its coordination number to four.
This is because the two aluminum ions bridge with two chloride ions, creating a more stable structure. The increased coordination number enhances the stability of the compound, as having more bonds generally means better stabilization.

• Coordination number 3: When AlCl₃ is not in dimer form, each aluminum atom is bonded to three chloride atoms.
• Coordination number 4: By forming a dimer (Al₂Cl₆), each aluminum can bond to a fourth chloride atom.

Understanding the concept of coordination numbers helps in grasping the stability and formation mechanisms of complex molecules like dimers.

ionization energy

Ionization energy is the amount of energy required to remove an electron from an atom. While it is an important property, it is not the main reason behind dimer formation in Aluminium (III) chloride.
High ionization energy usually signifies that an atom holds onto its electrons tightly, making it difficult to ionize. For aluminum, this is not directly related to its tendency to form dimers. Instead, ionization energy plays a more significant role in dictating the reactivity of elements and their ability or inability to form ions.
So, while understanding ionization energy is crucial in the context of chemical reactions and element behavior, it doesn't solely explain dimer formation in AlCl₃.

• High ionization energy: Energy required to remove an electron is high.
• Indirect influence: It affects how elements bond but isn't the direct cause of dimerization.

group III elements

Elements in Group III of the periodic table, such as aluminum, boron, gallium, and indium, are known for having three valence electrons. These three valence electrons make them trivalent, meaning they can form three covalent bonds.
Aluminium, being a Group III element, typically forms three bonds because of its three valence electrons. However, it's not just belonging to Group III that causes aluminum to form dimers. It is rather the way these valence electrons interact to achieve chemical stability that prompts such formations.

• Trivalent elements: Group III elements have three valence electrons.
• Chemical reactions: The interactions of these valence electrons contribute to the formation of various structures, including dimers.

Understanding the behavior of Group III elements provides a deeper insight into why aluminum prefers a certain bonding configuration, contributing indirectly to the dimer formation.

chemical stability

Chemical stability refers to the tendency of a compound to maintain its chemical structure and resist changes or decomposition. When Aluminium (III) chloride forms a dimer, it increases its chemical stability.
The formation of a dimer helps in lowering the overall energy of the system, making it more stable. By sharing chloride ions between two aluminum atoms, Al₂Cl₆ achieves a lower energy configuration compared to two separate AlCl₃ molecules.
The increased coordination number (from 3 to 4) also means that each aluminum atom is achieving a more stable electronic arrangement.

• Lower energy state: Dimer formation lowers the energy, increasing stability.
• Enhanced coordination: Helps achieve a more stable electronic structure.

Hence, the primary reason for the dimer formation in Aluminium (III) chloride is to achieve higher chemical stability through increased coordination.