Trigonal Planar Geometry

Definition

Trigonal planar geometry is a molecular geometry in which a central atom is covalently bonded to three other atoms, resulting in a flat, triangular arrangement around the central atom. This geometry is commonly observed in molecules with multiple bonds, such as those described in the context of Section 8.3: Multiple Bonds.

5 Must Know Facts For Your Next Test

1. Trigonal planar geometry is characterized by a central atom with three covalent bonds, resulting in a flat, triangular arrangement of the bonded atoms.
2. This geometry arises from the $sp^2$ hybridization of the central atom's orbitals, which allows for the formation of three equivalent $\sigma$ bonds and one $\pi$ bond.
3. Molecules with trigonal planar geometry, such as $\ce{BF3}$ and $\ce{CO2}$, often exhibit resonance, where the molecule can be represented by multiple, equally valid Lewis structures.
4. The trigonal planar geometry minimizes the repulsion between the three bonding electron pairs around the central atom, as predicted by the VSEPR theory.
5. Molecules with trigonal planar geometry typically have bond angles of approximately $120^\circ$ between the bonded atoms.

Review Questions

• Explain the relationship between hybridization and the trigonal planar geometry of a molecule.
  • The trigonal planar geometry of a molecule is a direct result of the $sp^2$ hybridization of the central atom's orbitals. In $sp^2$ hybridization, the central atom's s orbital and two of its p orbitals mix to form three equivalent $\sigma$ bonds, while the remaining p orbital forms a $\pi$ bond. This arrangement of three $\sigma$ bonds and one $\pi$ bond leads to the characteristic flat, triangular geometry around the central atom, with bond angles of approximately $120^\circ$.
• Describe how the VSEPR theory can be used to predict the trigonal planar geometry of a molecule.
  • The VSEPR theory states that molecules adopt a geometry that minimizes the repulsion between electron pairs in the valence shell of the central atom. In the case of a molecule with trigonal planar geometry, the central atom has three bonding electron pairs and no lone pairs. According to the VSEPR theory, this arrangement of three equivalent bonding electron pairs around the central atom results in a flat, triangular geometry that minimizes the repulsion between the electron pairs, leading to the observed trigonal planar structure.
• Analyze the role of resonance in molecules with trigonal planar geometry and explain how it contributes to the stability and bonding of the molecule.
  • Molecules with trigonal planar geometry, such as $\ce{CO2}$ and $\ce{BF3}$, often exhibit resonance, where the molecule can be represented by multiple, equally valid Lewis structures. This resonance stabilizes the molecule by distributing the $\pi$ bond electrons across multiple bonds, effectively increasing the bond order and reducing the overall energy of the system. The resonance also contributes to the delocalization of electrons, which enhances the stability of the molecule and its ability to participate in various chemical reactions. The trigonal planar geometry, combined with the resonance effect, is a crucial factor in determining the physical and chemical properties of these types of molecules.

Related terms

Valence Shell Electron Pair Repulsion (VSEPR) Theory:

A model used to predict the geometry of molecules based on the number of electron pairs around a central atom and the desire to minimize electron pair repulsion.

Hybridization:

The process of mixing atomic orbitals to form new, equivalent hybrid orbitals that can be used to describe the bonding in a molecule.

Resonance: The concept that a molecule can be represented by multiple, equally valid Lewis structures, which contribute to the overall bonding and stability of the molecule.