Question

How many of the following molecules have two lone pairs of electrons on central atom? SF\(_6\), BF\(_3\), ClF\(_3\), PCl\(_5\), BrF\(_5\), XeF\(_4\), H\(_2\)O, SF\(_4\)

• A. 5
• B. 4
• C. 3
• D. 2

Hint:

To find lone pairs on the central atom (A) in a molecule AX\(_n\)E\(_m\) (E=lone pair): 1. Find total valence electrons of the central atom (V). 2. Find number of electrons shared by bonds with surrounding atoms (X). For single bonds, this is usually the number of surrounding atoms if they need one electron each (like H, F, Cl). 3. Number of lone pair electrons = \(V - X\). 4. Number of lone pairs = \( (V-X)/2 \). Alternatively, using VSEPR theory: Total electron pairs = \( \frac{1}{2} (\text{Valence e}^- \text{ of central atom} + \text{No. of monovalent atoms} - \text{Charge on cation} + \text{Charge on anion}) \). Lone pairs = Total electron pairs - Number of surrounding atoms (bond pairs).

Answer 1

The Correct Option is C

Let's determine the number of lone pairs on the central atom for each molecule.
Valence electrons: S=6, B=3, Cl=7, P=5, Br=7, Xe=8, O=6, F=7, H=1.
1.
SF\(_6\): Central atom S.
Valence e\(^-\) of S = 6.
Bonds = 6 (with 6 F).
Total e\(^-\) pairs around S = 6.
Bonding pairs = 6.
Lone pairs = 6 - 6 = 0.
2.
BF\(_3\): Central atom B.
Valence e\(^-\) of B = 3.
Bonds = 3 (with 3 F).
Total e\(^-\) pairs around B = 3.
Bonding pairs = 3.
Lone pairs = 3 - 3 = 0.
(Incomplete octet) 3.
ClF\(_3\): Central atom Cl.
Valence e\(^-\) of Cl = 7.
Bonds = 3 (with 3 F).
Electrons used in bonding = 3.
Remaining valence e\(^-\) on Cl = 7 - 3 = 4.
Lone pairs = 4/2 = 2.
(Total e\(^-\) pairs = (7+3)/2 = 5.
Bonding pairs = 3.
Lone pairs = 5-3=2).
Shape: T-shaped.
4.
PCl\(_5\): Central atom P.
Valence e\(^-\) of P = 5.
Bonds = 5 (with 5 Cl).
Total e\(^-\) pairs around P = 5.
Bonding pairs = 5.
Lone pairs = 5 - 5 = 0.
5.
BrF\(_5\): Central atom Br.
Valence e\(^-\) of Br = 7.
Bonds = 5 (with 5 F).
Electrons used in bonding = 5.
Remaining valence e\(^-\) on Br = 7 - 5 = 2.
Lone pairs = 2/2 = 1.
(Total e\(^-\) pairs = (7+5)/2 = 6.
Bonding pairs = 5.
Lone pairs = 6-5=1).
Shape: Square pyramidal.
6.
XeF\(_4\): Central atom Xe.
Valence e\(^-\) of Xe = 8.
Bonds = 4 (with 4 F).
Electrons used in bonding = 4.
Remaining valence e\(^-\) on Xe = 8 - 4 = 4.
Lone pairs = 4/2 = 2.
(Total e\(^-\) pairs = (8+4)/2 = 6.
Bonding pairs = 4.
Lone pairs = 6-4=2).
Shape: Square planar.
7.
H\(_2\)O: Central atom O.
Valence e\(^-\) of O = 6.
Bonds = 2 (with 2 H).
Electrons used in bonding = 2.
Remaining valence e\(^-\) on O = 6 - 2 = 4.
Lone pairs = 4/2 = 2.
(Total e\(^-\) pairs = (6+2)/2 = 4.
Bonding pairs = 2.
Lone pairs = 4-2=2).
Shape: Bent.
8.
SF\(_4\): Central atom S.
Valence e\(^-\) of S = 6.
Bonds = 4 (with 4 F).
Electrons used in bonding = 4.
Remaining valence e\(^-\) on S = 6 - 4 = 2.
Lone pairs = 2/2 = 1.
(Total e\(^-\) pairs = (6+4)/2 = 5.
Bonding pairs = 4.
Lone pairs = 5-4=1).
Shape: See-saw.
Molecules with two lone pairs on the central atom are: - ClF\(_3\) - XeF\(_4\) - H\(_2\)O There are 3 such molecules.
This matches option (3).