The space model which is obtained by joining the points representing various bonded atoms gives the shape of the molecule. The geometry of the molecule is definite relative arrangement of the bonded atoms in a molecule. The shape and geometry of a molecule is explained by valence shell electron pair repulsion theory given by Gillespie and Nyholm.
Which is not the electron geometry of covalent molecules ?

Options

A. Pentagonal bipyramidal
B. Octahedral
C. Hexagonal
D. Tetrahedral

Answer

To determine which is not an electron geometry of covalent molecules according to the VSEPR (Valence Shell Electron Pair Repulsion) theory, we will analyze the given options: pentagonal bipyramidal, octahedral, hexagonal, and tetrahedral.

### Step-by-Step Solution:

1. **Understanding VSEPR Theory**:
- VSEPR theory states that the shape of a molecule is determined by the repulsion between electron pairs in the valence shell of the central atom. The geometries that arise from this theory are based on the number of bonding and lone pairs around the central atom.

2. **List of Common Geometries**:
- According to VSEPR theory, the common geometries for covalent molecules include:
- Linear
- Trigonal planar
- Tetrahedral
- Trigonal bipyramidal
- Octahedral
- Pentagonal bipyramidal
- These geometries are derived from the arrangement of electron pairs around the central atom.

3. **Analyzing the Options**:
- **Pentagonal Bipyramidal**: This geometry is possible and occurs when there are 7 bonding pairs (e.g., in certain transition metal complexes).
- **Octahedral**: This geometry is also possible with 6 bonding pairs.
- **Tetrahedral**: This geometry is possible with 4 bonding pairs.
- **Hexagonal**: This geometry is not recognized as a stable electron geometry in covalent molecules according to VSEPR theory.

4. **Reason for Hexagonal Geometry Not Being Possible**:
- In a hypothetical hexagonal geometry (AB6), the six atoms would be arranged in a plane. This arrangement leads to high electronic repulsion due to the close proximity of the atoms, resulting in steric hindrance. The electron pairs would experience significant repulsion, making this geometry unstable.

5. **Conclusion**:
- Therefore, the correct answer is that **hexagonal geometry is not an electron geometry of covalent molecules**.

### Final Answer:
**Hexagonal geometry is not an electron geometry of covalent molecules.**

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