Molecular Geometry

Concept: According to VSEPR theory, molecular geometry depends on the number of bond pairs and lone pairs around the central atom.

Step 1: Analyze each molecule.

• : Octahedral geometry
• : Trigonal bipyramidal
• : Trigonal planar
• : Linear
• : 4 bond pairs, no lone pairs Tetrahedral

Step 2: Select correct option.
Only has tetrahedral geometry.

Concept: Expanded octet occurs when the central atom has more than 8 electrons in its valence shell. This is possible for elements in period 3 or beyond due to availability of d-orbitals.

Step 1: Analyze each compound.

• : Sulfur has 8 electrons no expansion
• : Nitrogen cannot expand octet
• : Nitrogen follows octet
• : Phosphorus has 8 electrons
• : Sulfur has more than 8 electrons (expanded octet)

Step 2: Select correct compound.
Only shows expanded octet.

Concept: Use VSEPR theory.

Step 1: Check .
Given option says trigonal bipyramidal → incorrect.

Concept: Dipole moment depends on geometry and electronegativity.

Step 1: Order reasoning.

Step 1: Understanding the Concept:
The shape of a molecule is determined by the Valence Shell Electron Pair Repulsion (VSEPR) theory, which considers both bond pairs and lone pairs of electrons around the central atom.

Step 2: Detailed Explanation:
Analyze the shape of each species:
• SF : Central S has 5 electron pairs (4 bond pairs, 1 lone pair). Shape is see-saw.
• NH : Central N has 4 bond pairs, no lone pairs. Shape is tetrahedral.
• CH Cl : Central C has 4 bond pairs. Shape is tetrahedral.
• CH : Central C has 4 bond pairs. Shape is tetrahedral.
• XeF : Central Xe has 6 electron pairs (4 bond pairs, 2 lone pairs). The two lone pairs occupy opposite positions, giving a square planar shape.

Step 3: Final Answer:
XeF has a square planar shape.

Step 1: Understanding the Concept:
A molecule is nonpolar if the vector sum of all bond dipoles is zero. This requires a symmetrical geometry.

Step 2: Detailed Explanation:

• CO : Linear geometry (O=C=O). The two C=O bond dipoles are equal in magnitude and opposite in direction. They cancel out. Nonpolar.
• H O: Bent geometry. The two O-H bond dipoles do not cancel. Polar.
• CH OH: Methanol has a polar O-H bond and a bent structure around oxygen. Polar.
• HCl: Diatomic molecule with a polar bond. Polar.
• NaCl: Ionic compound, not a covalent molecule. In the gaseous state, it exists as a polar diatomic molecule.

Step 3: Final Answer:
CO is the nonpolar molecule.

Step 1: Understanding the Concept:
To find the total number of electrons around a central atom in a molecule, we need to draw its Lewis dot structure and count the valence electrons surrounding it. This includes all electrons shared in bonds (2 electrons per single bond) plus any unshared non-bonding electrons (lone pairs).

Step 2: Key Formula or Approach:
For each molecule:
1. Identify the central atom and its number of valence electrons.
2. Determine the number of single bonds formed with surrounding atoms.
3. Calculate remaining lone pair electrons on the central atom.
4. Total electrons = (Number of bonds 2) + (Number of lone pair electrons).

Step 3: Detailed Explanation:
Let's analyze each molecule:
1. (Phosphorus pentachloride):
- Central atom: Phosphorus (P), Group 15 5 valence electrons.
- It forms 5 single bonds with 5 Chlorine atoms.
- All 5 valence electrons are used in bonding. Lone pairs = 0.
- Total electrons = electrons.
(Note: This is an expanded octet).
2.
- Central atom: Sulfur (S), Group 16 6 valence electrons.
- It forms 6 single bonds with 6 Fluorine atoms.
- All 6 valence electrons are used in bonding. Lone pairs = 0.
- Total electrons = electrons.
(Note: This is an expanded octet).
3. (Sulfur dichloride):
- Central atom: Sulfur (S), Group 16 6 valence electrons.
- It forms 2 single bonds with 2 Chlorine atoms (using 2 valence electrons).
- Remaining valence electrons on S = electrons (which is 2 lone pairs).
- Total electrons around S = (2 bonding pairs 2) + 4 lone pair electrons = electrons.

Listing them in the requested order: 10, 12, 8.

Step 4: Final Answer:
The total number of electrons are 10, 12, and 8 respectively.

Step 1: Understanding the Concept:
A molecule has a zero dipole moment if it is perfectly symmetrical, such that the individual bond dipoles cancel each other out. For an type molecule to have zero dipole moment, it must have a trigonal planar geometry with no lone pairs on the central atom.

Step 2: Detailed Explanation:
1. Nitrogen (N): In , nitrogen has 5 valence electrons. It forms 3 bonds and has 1 lone pair. Geometry is Pyramidal. Dipoles do not cancel ( ).
2. Chlorine/Bromine (Cl/Br): In , the central atom has 7 valence electrons. It forms 3 bonds and has 2 lone pairs. Geometry is T-shaped. Dipoles do not cancel ( ).
3. Boron (B): In , boron has 3 valence electrons. It forms 3 bonds and has 0 lone pairs. Geometry is Trigonal Planar (120° angles). All bond dipoles cancel perfectly.
4. Carbon (C): Carbon typically forms 4 bonds ( ). A neutral molecule would be a radical and not a stable neutral molecule in this context.

Step 3: Final Answer
The element X is boron, forming the symmetrical molecule.

Concept:
• VSEPR theory determines molecular geometry

Step 1: Total electron pairs


Step 2: Geometry
Electron geometry → trigonal bipyramidal
Lone pairs occupy equatorial positions

Step 3: Molecular shape
Remaining atoms form T-shape Final Conclusion:
Option (A)

Concept:
• structure and VSEPR theory

Step 1: Geometry
Trigonal bipyramidal → correct

Step 2: Bond arrangement
3 equatorial + 2 axial → correct

Step 3: Check statements
(iii) False (axial suffer more repulsion)
(iv) False (axial bonds weaker)
(v) True ( hybridization) Final Conclusion:
Option (E)