Hallides of boron and aluminium are Lewis Acids. Assign reason.

Options

A. Both halides `(MX_(3))` can accept electrons from a donor to complete their octet.
B. Both halides `(MX_(3))` can donate a pair of electrons
C. Both halides `(MX_(3))` are covalent polymeric structures.
D. Both halides `(MX_(3))` react with water to give hydroxides and HCI

Explanation

**Step-by-Step Solution:**

1. **Understanding Lewis Acids**:
- Lewis acids are defined as substances that can accept a pair of electrons to form a covalent bond. This property is crucial in understanding why certain compounds behave as Lewis acids.

2. **Examining Boron Halides (BX3)**:
- Boron typically forms halides of the type BX3 (e.g., BF3, BCl3).
- Boron has an electronic configuration of 1s² 2s² 2p¹. In its ground state, it has three valence electrons.
- When boron forms BX3, it undergoes excitation where one of the 2s electrons is promoted to the 2p orbital, resulting in the configuration suitable for bonding.

3. **Vacant Orbitals in Boron**:
- After forming three covalent bonds with halogens, boron has an empty p-orbital. This vacant orbital allows boron to accept a lone pair of electrons from a Lewis base, making it a Lewis acid.

4. **Examining Aluminium Halides (AlX3)**:
- Aluminium, like boron, forms halides of the type AlX3 (e.g., AlCl3).
- Aluminium has an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p¹. Similar to boron, it has three valence electrons.
- In the excited state, one of the 3s electrons is promoted to the 3p orbital, allowing it to form three covalent bonds with halogens.

5. **Vacant Orbitals in Aluminium**:
- After bonding, aluminium also has a vacant p-orbital. This vacant orbital enables aluminium to accept a lone pair from a Lewis base, confirming its status as a Lewis acid.

6. **Conclusion**:
- Both boron and aluminium halides can accept electron pairs due to the presence of vacant orbitals after forming covalent bonds. This characteristic is what classifies them as Lewis acids.

**Final Statement**:
Thus, the halides of boron and aluminium are Lewis acids because they have vacant orbitals that allow them to accept electron pairs from Lewis bases.

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