Trigonal planar geometry

Definition

Trigonal planar geometry refers to a molecular shape where a central atom is bonded to three other atoms, arranged in a flat, triangular formation. This arrangement results in bond angles of approximately 120 degrees and is characteristic of molecules with a steric number of three, meaning there are three regions of electron density around the central atom, such as in the case of boron trifluoride (BF₃).

5 Must Know Facts For Your Next Test

1. Trigonal planar geometry typically occurs in molecules with a central atom that has no lone pairs and three bonded atoms.
2. The symmetrical arrangement in trigonal planar geometry minimizes electron pair repulsions, stabilizing the molecule.
3. Common examples of trigonal planar molecules include BF₃, AlCl₃, and formaldehyde (CH₂O).
4. This geometry is an important concept in determining molecular polarity; if the surrounding atoms are different, the molecule may be polar despite its symmetrical shape.
5. Trigonal planar geometry can be influenced by the presence of electronegative atoms, which may affect bond angles and molecular behavior.

Review Questions

• How does VSEPR theory explain the trigonal planar geometry observed in certain molecules?
  • VSEPR theory explains trigonal planar geometry by stating that electron pairs around a central atom will arrange themselves to minimize repulsions. In the case of a central atom bonded to three other atoms with no lone pairs, these bonding pairs spread out evenly in a plane, forming 120-degree bond angles. This arrangement stabilizes the molecule by reducing electron-electron repulsion.
• What role does steric number play in determining whether a molecule adopts trigonal planar geometry?
  • The steric number is crucial for determining molecular shape because it represents the total number of electron regions around a central atom. A steric number of three indicates that there are three regions of electron density, which corresponds to trigonal planar geometry. If there were additional lone pairs present, the geometry could change to something like bent or trigonal pyramidal instead.
• Evaluate how the presence of different atoms in a trigonal planar molecule can affect its overall polarity.
  • In a trigonal planar molecule with different surrounding atoms, the polarity can vary significantly due to differences in electronegativity. If all three substituents are identical, such as in BF₃, the molecule is nonpolar because the dipoles cancel each other out. However, if one of the substituents is different, like in CH₂O, this creates an unequal distribution of electron density, resulting in a net dipole moment and making the molecule polar. Thus, while trigonal planar geometry suggests symmetry, the actual arrangement can lead to diverse polarity outcomes based on constituent atoms.

Related terms

VSEPR Theory: A model used to predict the shape of individual molecules based on the repulsion between electron pairs surrounding the central atom.

Bond Angle: The angle formed between two bonds that originate from the same atom, crucial for understanding molecular geometry.

Steric Number: The total number of electron pairs (bonding and lone pairs) around a central atom that determines its molecular geometry.