Which compound given is not a Lewis acid ?

Options

A. `BF_3`
B. `FeCl_3`
C. `AlCl_3`
D. `NH_3`

Answer

To determine which compound is not a Lewis acid, we need to understand the definition of a Lewis acid. A Lewis acid is defined as a species that can accept an electron pair. This typically includes cations and molecules with incomplete octets or vacant orbitals.

Let's analyze the compounds mentioned in the video transcript step by step:

### Step 1: Analyze BF3 (Boron Trifluoride)
- **Valence Electrons**: Boron has 3 valence electrons, and each fluorine has 7. Therefore, BF3 has a total of 6 valence electrons (3 from B + 3×7 from F).
- **Octet**: Boron has only 6 electrons in its outer shell, so its octet is incomplete.
- **Vacant Orbitals**: BF3 has a vacant p-orbital, allowing it to accept an electron pair.
- **Conclusion**: BF3 is a Lewis acid.

### Step 2: Analyze FeCl3 (Iron(III) Chloride)
- **Oxidation State**: Iron in FeCl3 is in the +3 oxidation state.
- **Valence Electrons**: The electron configuration of Fe in +3 state is [Ar] 4s² 3d⁵. With 3 Cl atoms donating 6 electrons, it can accept these electrons to complete its octet.
- **Conclusion**: FeCl3 acts as a Lewis acid.

### Step 3: Analyze AlCl3 (Aluminum Chloride)
- **Valence Electrons**: Aluminum has 3 valence electrons, and with 3 Cl atoms, AlCl3 has a total of 6 valence electrons.
- **Octet**: Aluminum also has an incomplete octet.
- **Vacant Orbitals**: AlCl3 can accept electron pairs from Lewis bases.
- **Conclusion**: AlCl3 is a Lewis acid.

### Step 4: Analyze NH3 (Ammonia)
- **Valence Electrons**: Nitrogen has 5 valence electrons, and each hydrogen has 1, giving NH3 a total of 8 valence electrons.
- **Octet**: The octet for nitrogen is complete.
- **Lone Pair**: NH3 has a lone pair of electrons that it can donate.
- **Conclusion**: NH3 acts as a Lewis base, not a Lewis acid.

### Final Conclusion
Among the compounds analyzed, **NH3 (Ammonia)** is not a Lewis acid; instead, it is a Lewis base because it donates an electron pair.

### Answer
The compound that is not a Lewis acid is **NH3 (Ammonia)**.

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