Which of the followings are Lewis acids: `H_(2)O, BF_(3), H^(o+)` and `NH_(4)`?

Explanation

To determine which of the given species are Lewis acids, we first need to understand the definition of a Lewis acid. A Lewis acid is defined as a substance that can accept an electron pair from a Lewis base. This typically means that Lewis acids are electron-deficient species.

Let's analyze each of the given species one by one:

1. **H₂O (Water)**:
- Water has a central oxygen atom that has two lone pairs of electrons.
- Since water can donate these lone pairs, it acts as a Lewis base rather than a Lewis acid.
- **Conclusion**: H₂O is **not** a Lewis acid.

2. **BF₃ (Boron Trifluoride)**:
- Boron trifluoride has a boron atom that is surrounded by three fluorine atoms.
- Boron has only six electrons in its valence shell (incomplete octet), making it electron-deficient.
- This deficiency allows BF₃ to accept an electron pair, qualifying it as a Lewis acid.
- **Conclusion**: BF₃ **is** a Lewis acid.

3. **H⁺ (Hydrogen Ion)**:
- The hydrogen ion (H⁺) is simply a proton and has no electrons.
- It is highly electron-deficient and can readily accept an electron pair.
- Therefore, H⁺ qualifies as a Lewis acid.
- **Conclusion**: H⁺ **is** a Lewis acid.

4. **NH₄⁺ (Ammonium Ion)**:
- The ammonium ion has a positive charge and is formed by the addition of a proton to ammonia (NH₃).
- It can accept an electron pair due to its positive charge, which makes it electron-deficient.
- Thus, NH₄⁺ also qualifies as a Lewis acid.
- **Conclusion**: NH₄⁺ **is** a Lewis acid.

### Final Summary:
- **Lewis Acids**: BF₃, H⁺, NH₄⁺
- **Not a Lewis Acid**: H₂O