Which of the following can not act as a bronsted base?

Options

A. `NH_(3)`
B. `BF_(3)`
C. `H_(3)O^(+)`
D. `H_(2)O`

Answer

To determine which of the following substances cannot act as a Brønsted base, we need to understand the definition of a Brønsted base. A Brønsted base is a substance that can accept a proton (H⁺).

Let's analyze each option step by step:

### Step 1: Understanding Brønsted Bases
- A Brønsted base is defined as a proton acceptor. Therefore, to identify which substance cannot act as a Brønsted base, we need to check if the substance can accept a proton.

### Step 2: Analyze Each Option
1. **NH₃ (Ammonia)**:
- Ammonia has a lone pair of electrons on the nitrogen atom. It can accept a proton (H⁺) to form NH₄⁺ (ammonium ion).
- Conclusion: NH₃ can act as a Brønsted base.

2. **BF₃ (Boron Trifluoride)**:
- BF₃ is electron-deficient; it has only six electrons in its valence shell. It does not have a lone pair of electrons to accept a proton.
- Conclusion: BF₃ cannot act as a Brønsted base.

3. **H₃O⁺ (Hydronium Ion)**:
- H₃O⁺ has a positive charge and can donate a proton to form water (H₂O). However, it is not a proton acceptor; it is already a protonated species.
- Conclusion: H₃O⁺ cannot act as a Brønsted base.

4. **H₂O (Water)**:
- Water can accept a proton to form H₃O⁺. Thus, it can act as a Brønsted base.
- Conclusion: H₂O can act as a Brønsted base.

### Step 3: Final Conclusion
From the analysis, we find that:
- **NH₃** can act as a Brønsted base.
- **BF₃** cannot act as a Brønsted base.
- **H₃O⁺** cannot act as a Brønsted base.
- **H₂O** can act as a Brønsted base.

Thus, the substances that cannot act as a Brønsted base are **BF₃** and **H₃O⁺**.

### Answer:
**BF₃ and H₃O⁺ cannot act as Brønsted bases.**

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