Which of the following molecule is planar due to back bonding

Options

A. `NCl_(3)`
B. `PF_(3)`
C. `BF_(3)`
D. None

Answer

To determine which molecule is planar due to back bonding, we need to analyze the conditions required for back bonding and examine the given molecules. Here’s a step-by-step solution:

### Step 1: Understand Back Bonding
Back bonding occurs when there is a donation of electron density from a filled orbital of a surrounding atom (which has lone pairs) to an empty orbital of the central atom. For back bonding to be possible, two main conditions must be met:
1. The central atom must have vacant orbitals.
2. The surrounding atoms must have lone pairs.

### Step 2: Identify the Molecules
Let's consider the following molecules:
- NCl3 (Nitrogen trichloride)
- PF3 (Phosphorus trifluoride)
- BF3 (Boron trifluoride)

### Step 3: Analyze NCl3
- **Central Atom**: Nitrogen (N)
- **Valence Electrons**: Nitrogen has 5 valence electrons (2s² 2p³).
- **Bonding**: Nitrogen forms 3 bonds with chlorine atoms, using 3 of its valence electrons.
- **Lone Pairs**: Nitrogen has 2 electrons left as a lone pair.
- **Vacant Orbitals**: Nitrogen does not have any vacant orbitals available for back bonding.

**Conclusion**: NCl3 does not exhibit back bonding.

### Step 4: Analyze PF3
- **Central Atom**: Phosphorus (P)
- **Valence Electrons**: Phosphorus has 5 valence electrons (3s² 3p³).
- **Bonding**: Phosphorus forms 3 bonds with fluorine atoms.
- **Lone Pairs**: Each fluorine atom has 3 lone pairs.
- **Vacant Orbitals**: Phosphorus has vacant d orbitals available.

**Conclusion**: While PF3 has the conditions for back bonding, phosphorus and fluorine belong to different periods (3rd and 2nd, respectively), which leads to poor overlap and limits back bonding.

### Step 5: Analyze BF3
- **Central Atom**: Boron (B)
- **Valence Electrons**: Boron has 3 valence electrons (2s² 2p¹).
- **Bonding**: Boron forms 3 bonds with fluorine atoms.
- **Lone Pairs**: Each fluorine atom has 3 lone pairs.
- **Vacant Orbitals**: Boron has a vacant p orbital available for back bonding.

**Conclusion**: Boron and fluorine are from the same period (2nd), allowing for effective overlap and back bonding. Therefore, BF3 is planar due to back bonding.

### Final Answer
The molecule that is planar due to back bonding is **BF3**.