Which of the following has highest proton affinity?

Options

A. `NH_(3)`
B. `PH_(3)`
C. `H_(2)O`
D. `H_(2)S`

Explanation

To determine which of the given compounds has the highest proton affinity, we will analyze the proton affinity of each compound based on their ability to donate electron pairs.

### Step-by-Step Solution:

1. **Understanding Proton Affinity**:
- Proton affinity refers to the tendency of a chemical species to attract and bond with protons (H⁺ ions). Compounds with a high tendency to donate electron pairs generally exhibit high proton affinity.

2. **Analyzing Ammonia (NH₃)**:
- Ammonia has a nitrogen atom with a lone pair of electrons. The nitrogen atom is relatively small, allowing the electron density to be concentrated in a small region. This high electron density makes ammonia a strong Lewis base, meaning it can effectively donate its lone pair to a proton, forming the ammonium ion (NH₄⁺).

3. **Comparing with Phosphine (PH₃)**:
- Phosphine has a phosphorus atom, which is larger than nitrogen. The lone pair of electrons in PH₃ is spread over a larger volume, leading to lower electron density compared to ammonia. Additionally, phosphorus has vacant d-orbitals, which can lead to electron delocalization and reduce the availability of the lone pair for donation. Therefore, PH₃ has a lower proton affinity than NH₃.

4. **Comparing with Water (H₂O)**:
- Water has an oxygen atom that is more electronegative than nitrogen. This high electronegativity means that oxygen holds onto its electrons more tightly, making it less willing to donate its lone pairs compared to nitrogen in ammonia. Thus, water has a lower proton affinity than ammonia.

5. **Comparing with Sulfur Compounds**:
- Sulfur, being larger than nitrogen, has its electron density spread over a larger area, which makes it less effective in donating electrons. Therefore, sulfur compounds would also exhibit lower proton affinity compared to ammonia.

6. **Conclusion**:
- After comparing ammonia with phosphine, water, and sulfur compounds, it is clear that ammonia (NH₃) has the highest proton affinity among the options given.

### Final Answer:
**Ammonia (NH₃) has the highest proton affinity.**