The number of three centre two electron bonds in a molecule of diborane is________.

Explanation

To determine the number of three-center two-electron bonds in a molecule of diborane (B₂H₆), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of Diborane**:
- Diborane consists of two boron (B) atoms and six hydrogen (H) atoms. The molecular formula is B₂H₆.

2. **Identify the Bonding in Diborane**:
- In diborane, each boron atom is bonded to two hydrogen atoms and shares a bond with the other boron atom through a unique bonding arrangement.

3. **Define Three-Center Two-Electron Bonds**:
- A three-center two-electron bond involves three atomic nuclei (in this case, two boron nuclei and one hydrogen nucleus) sharing two electrons. This type of bond is often referred to as a "banana bond" due to its bent shape.

4. **Count the Three-Center Two-Electron Bonds**:
- In diborane, there are two such bonds. Each boron atom forms a three-center two-electron bond with one hydrogen atom and the other boron atom. Therefore, we have:
- One three-center two-electron bond involving B1, B2, and H1.
- Another three-center two-electron bond involving B1, B2, and H2.

5. **Conclusion**:
- Thus, the total number of three-center two-electron bonds in a molecule of diborane is **2**.

### Final Answer:
The number of three-center two-electron bonds in a molecule of diborane is **2**.

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