Aluminium (III) chloride forms a dimer because aluminium

Options

A. Belongs to `3^(rd)` group
B. Can have higher coordination number
C. Cannot form a trimer
D. Has high ionization energy

Answer

To determine why Aluminium (III) chloride forms a dimer, we need to analyze the electronic configuration of aluminium and its bonding characteristics with chlorine. Here’s a step-by-step solution:

### Step 1: Understand the Electronic Configuration of Aluminium
Aluminium (Al) has an atomic number of 13. Its electronic configuration is:
\[ \text{Al: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^1 \]
This shows that aluminium has three electrons in its outermost shell (3s and 3p orbitals).

**Hint:** Remember that the outermost electrons play a crucial role in bonding and determining the stability of the compound.

### Step 2: Hybridization of Aluminium
When aluminium forms Aluminium chloride (AlCl₃), it undergoes hybridization. One of the 3s electrons is promoted to the 3p orbital, resulting in:
\[ 3s^0 \, 3p^2 \]
This configuration allows aluminium to have one vacant p orbital available for bonding.

**Hint:** Hybridization helps in understanding how atoms can form bonds by utilizing their orbitals.

### Step 3: Formation of Aluminium Chloride
Aluminium can bond with three chlorine atoms, each contributing one electron. The structure of AlCl₃ is trigonal planar, but it does not have a complete octet around aluminium (it has only six electrons).

**Hint:** An incomplete octet indicates that the atom may seek to bond further to achieve stability.

### Step 4: Dimerization of Aluminium Chloride
To achieve a stable octet, AlCl₃ molecules can pair up to form a dimer, Al₂Cl₆. In this dimer, each aluminium atom shares its vacant p orbital with a chlorine atom from another AlCl₃ molecule, forming coordinate covalent bonds.

**Hint:** Dimerization is a common way for molecules to achieve stability by sharing electrons.

### Step 5: Coordination Number
In the dimer Al₂Cl₆, the coordination number of aluminium increases to 4, as each aluminium atom is surrounded by four chlorine atoms. This higher coordination number is possible due to the presence of vacant orbitals in aluminium.

**Hint:** Coordination number is an important concept in understanding the geometry and bonding in coordination compounds.

### Conclusion
The reason Aluminium (III) chloride forms a dimer is that aluminium can have a high coordination number due to the presence of vacant orbitals, allowing it to bond with more chlorine atoms and achieve a stable electron configuration.

**Final Answer:** Aluminium (III) chloride forms a dimer because aluminium can have a high coordination number.

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