Number of electron-rich hydrides among the following are :
`CH_(4),NH_(3),PH_(3),H_(2)O,H_(2)S,BH_(3),HF,AlH_(3),AsH_(3)`.

Explanation

To determine the number of electron-rich hydrides among the given compounds, we need to analyze each hydride based on the presence of lone pairs of electrons on the central atom. Electron-rich hydrides are those that have a higher electron density due to the presence of lone pairs.

Let's analyze each hydride one by one:

1. **CH₄ (Methane)**:
- Central atom: Carbon
- Carbon has no lone pairs and forms four bonds with hydrogen.
- **Electron-rich?** No

2. **NH₃ (Ammonia)**:
- Central atom: Nitrogen
- Nitrogen has one lone pair and forms three bonds with hydrogen.
- **Electron-rich?** Yes

3. **PH₃ (Phosphine)**:
- Central atom: Phosphorus
- Phosphorus has one lone pair and forms three bonds with hydrogen.
- **Electron-rich?** Yes

4. **H₂O (Water)**:
- Central atom: Oxygen
- Oxygen has two lone pairs and forms two bonds with hydrogen.
- **Electron-rich?** Yes

5. **H₂S (Hydrogen Sulfide)**:
- Central atom: Sulfur
- Sulfur has two lone pairs and forms two bonds with hydrogen.
- **Electron-rich?** Yes

6. **BH₃ (Boron Hydride)**:
- Central atom: Boron
- Boron has no lone pairs and forms three bonds with hydrogen.
- **Electron-rich?** No

7. **HF (Hydrogen Fluoride)**:
- Central atom: Fluorine
- Fluorine has three lone pairs and forms one bond with hydrogen.
- **Electron-rich?** Yes

8. **AlH₃ (Aluminium Hydride)**:
- Central atom: Aluminium
- Aluminium has no lone pairs and forms three bonds with hydrogen.
- **Electron-rich?** No

9. **AsH₃ (Arsenic Hydride)**:
- Central atom: Arsenic
- Arsenic has one lone pair and forms three bonds with hydrogen.
- **Electron-rich?** Yes

Now, let's summarize the findings:

- Electron-rich hydrides: NH₃, PH₃, H₂O, H₂S, HF, AsH₃
- Total number of electron-rich hydrides = 6

**Final Answer: 6**