In which of the following species central atom is NOT surrounded by exactly 8 valence electrons ?

Options

A. `BF_(4)^(-)`
B. `NCl_(3)`
C. `PCl_(4)^(+)`
D. `SF_(4)`

Explanation

To determine which species has a central atom that is not surrounded by exactly 8 valence electrons, we will analyze each option step by step.

### Step 1: Analyze BF4^-
- **Valence Electrons Calculation**: Boron (B) has 3 valence electrons, and each fluorine (F) has 7 valence electrons. There are 4 fluorine atoms.
- **Total Valence Electrons**: 3 (B) + 4 × 7 (F) = 3 + 28 = 31 electrons.
- **Lewis Structure**: Boron forms 3 single bonds with 3 fluorine atoms and a coordinate bond with the fourth fluorine.
- **Valence Electrons around Boron**: Boron shares 1 electron with each of the 3 fluorine atoms (3 electrons) and receives 2 electrons from the coordinate bond (1 from the fourth fluorine). Thus, Boron has 8 electrons around it.
- **Conclusion**: Boron in BF4^- is surrounded by 8 valence electrons.

### Step 2: Analyze NCl3
- **Valence Electrons Calculation**: Nitrogen (N) has 5 valence electrons, and each chlorine (Cl) has 7 valence electrons. There are 3 chlorine atoms.
- **Total Valence Electrons**: 5 (N) + 3 × 7 (Cl) = 5 + 21 = 26 electrons.
- **Lewis Structure**: Nitrogen forms 3 single bonds with chlorine atoms.
- **Valence Electrons around Nitrogen**: Nitrogen shares 1 electron with each chlorine (3 electrons), giving it a total of 8 electrons around it.
- **Conclusion**: Nitrogen in NCl3 is surrounded by 8 valence electrons.

### Step 3: Analyze PCl4^+
- **Valence Electrons Calculation**: Phosphorus (P) has 5 valence electrons, and each chlorine (Cl) has 7 valence electrons. There are 4 chlorine atoms.
- **Total Valence Electrons**: 5 (P) + 4 × 7 (Cl) + 1 (positive charge) = 5 + 28 + 1 = 34 electrons.
- **Lewis Structure**: Phosphorus forms 4 bonds with chlorine atoms.
- **Valence Electrons around Phosphorus**: Phosphorus shares 1 electron with each chlorine (4 electrons), giving it a total of 8 electrons around it.
- **Conclusion**: Phosphorus in PCl4^+ is surrounded by 8 valence electrons.

### Step 4: Analyze SF4
- **Valence Electrons Calculation**: Sulfur (S) has 6 valence electrons, and each fluorine (F) has 7 valence electrons. There are 4 fluorine atoms.
- **Total Valence Electrons**: 6 (S) + 4 × 7 (F) = 6 + 28 = 34 electrons.
- **Lewis Structure**: Sulfur forms 4 bonds with fluorine atoms and has 1 lone pair.
- **Valence Electrons around Sulfur**: Sulfur shares 1 electron with each fluorine (4 electrons) and has 2 electrons in the lone pair, giving it a total of 10 electrons around it.
- **Conclusion**: Sulfur in SF4 is surrounded by 10 valence electrons.

### Final Conclusion:
The species in which the central atom is NOT surrounded by exactly 8 valence electrons is **SF4**.