In which of the following back - bonding is possible

Options

A. `BF_3`
B. `(SiH_3)_3N`
C. `NH_3`
D. both A and B

Answer

To determine in which of the given compounds back-bonding is possible, we will analyze each compound step-by-step.

### Step 1: Analyze BF3 (Boron Trifluoride)
- **Structure**: Boron (B) is bonded to three fluorine (F) atoms.
- **Electron Configuration**: Boron has 3 valence electrons, while each fluorine has 7 valence electrons.
- **Lone Pairs**: Each fluorine has 3 lone pairs of electrons.
- **Back-Bonding**: Boron is electron-deficient (it has only 6 electrons in its valence shell). The fluorine atoms can donate a pair of electrons from their lone pairs into the empty p-orbital of boron, resulting in back-bonding.

**Conclusion**: Back-bonding occurs in BF3.

### Step 2: Analyze SiH3-N (Silicon Hydride and Nitrogen)
- **Structure**: Silicon (Si) is bonded to three hydrogen (H) atoms and one nitrogen (N) atom.
- **Electron Configuration**: Silicon has 4 valence electrons, and nitrogen has 5 valence electrons.
- **Lone Pairs**: Nitrogen has one lone pair of electrons.
- **Back-Bonding**: The lone pair on nitrogen can be shared with silicon, allowing for back-bonding. This interaction gives a double bond character between silicon and nitrogen.

**Conclusion**: Back-bonding occurs in SiH3-N.

### Step 3: Analyze NH3 (Ammonia)
- **Structure**: Nitrogen (N) is bonded to three hydrogen (H) atoms.
- **Electron Configuration**: Nitrogen has 5 valence electrons, and hydrogen has 1 valence electron.
- **Lone Pairs**: Nitrogen has one lone pair of electrons.
- **Back-Bonding**: Hydrogen does not have any empty orbitals to accept electrons from nitrogen. Therefore, nitrogen cannot transfer electrons to hydrogen.

**Conclusion**: No back-bonding occurs in NH3.

### Final Conclusion
Based on the analysis:
- Back-bonding occurs in **BF3** and **SiH3-N**.
- Back-bonding does not occur in **NH3**.

Thus, the correct answer to the question is that back-bonding is possible in the first and second compounds.