Why do boron halides form addition compounds with ammonia ?

Explanation

**Step-by-Step Solution:**

1. **Identify the Compounds**: The question involves boron halides, specifically boron trifluoride (BF3), and ammonia (NH3).

2. **Structure of Boron Trifluoride (BF3)**: Boron trifluoride consists of one boron atom bonded to three fluorine atoms. The boron atom has only six electrons in its valence shell, making it electron deficient. This means that BF3 does not have a complete octet and is classified as a Lewis acid.

3. **Structure of Ammonia (NH3)**: Ammonia consists of one nitrogen atom bonded to three hydrogen atoms. The nitrogen atom has a lone pair of electrons, making NH3 an electron-rich species. Therefore, ammonia acts as a Lewis base.

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