Which among the following is a Lewis acid?

Options

A. `NH_3`
B. `BF_3`
C. `H_2O`
D. `NH_4`

Answer

To determine which among the given options is a Lewis acid, we need to understand the definition of a Lewis acid. A Lewis acid is defined as a substance that can accept an electron pair, often due to the presence of a vacant orbital or an incomplete octet.

Let's analyze each option step by step:

1. **Ns3 (Ammonia, NH3)**:
- Ammonia has a nitrogen atom that is surrounded by three hydrogen atoms.
- The nitrogen has one lone pair of electrons, giving it a total of 8 electrons in its valence shell (2 from the lone pair and 6 from the three N-H bonds).
- Since it has a complete octet and can donate a pair of electrons, it acts as a Lewis base, not a Lewis acid.

**Hint**: Check if the molecule has a complete octet and lone pairs; if it does, it may be a Lewis base.

2. **Bf3 (Boron Trifluoride)**:
- Boron trifluoride consists of a boron atom bonded to three fluorine atoms.
- Boron has only 6 electrons in its valence shell (3 from the B-F bonds), which means it does not have a complete octet.
- The boron atom has a vacant p orbital, allowing it to accept an electron pair. Therefore, BF3 qualifies as a Lewis acid.

**Hint**: Look for incomplete octets or vacant orbitals; these are indicators of a Lewis acid.

3. **H2 (Hydrogen Molecule)**:
- The hydrogen molecule consists of two hydrogen atoms sharing a pair of electrons.
- Each hydrogen atom has a complete duet (2 electrons), and there are no vacant orbitals available for accepting an electron pair.
- Thus, H2 does not act as a Lewis acid.

**Hint**: Consider whether the molecule can accept additional electrons; if it cannot, it is likely not a Lewis acid.

4. **NH4 (Ammonium Ion)**:
- The ammonium ion has a nitrogen atom bonded to four hydrogen atoms.
- The nitrogen has 8 electrons in its valence shell (4 from the N-H bonds) and does not have any lone pairs.
- Since it has a complete octet and cannot accept any more electron pairs, NH4 is not a Lewis acid.

**Hint**: Analyze the electron count and lone pairs; a complete octet with no vacant orbitals indicates a lack of Lewis acidity.

**Conclusion**: Among the options provided, **Bf3 (Boron Trifluoride)** is the only Lewis acid because it can accept an electron pair due to its incomplete octet and vacant orbital.

**Final Answer**: Bf3