Trigonal bipyramidal

Definition

Trigonal bipyramidal is a molecular geometry formed when a central atom is bonded to five other atoms, resulting in a shape that has three atoms in a plane (equatorial positions) and two atoms above and below this plane (axial positions). This arrangement allows for optimal spacing between the bonded atoms, minimizing repulsion and maximizing stability based on the principles of molecular shapes.

5 Must Know Facts For Your Next Test

1. In a trigonal bipyramidal geometry, the bond angles are 120 degrees for equatorial atoms and 90 degrees between axial and equatorial atoms.
2. Common examples of molecules with trigonal bipyramidal geometry include phosphorus pentachloride (PCl5) and sulfur hexafluoride (SF6).
3. The central atom in a trigonal bipyramidal structure typically has five valence electrons that are involved in bonding.
4. When considering lone pairs, the presence of lone pairs can alter the ideal trigonal bipyramidal shape, leading to different geometries such as seesaw or T-shaped.
5. Trigonal bipyramidal geometries are commonly found in transition metals and heavier main group elements due to their ability to expand their valence shell.

Review Questions

• How does VSEPR theory explain the trigonal bipyramidal geometry, and what factors contribute to this shape?
  • VSEPR theory explains the trigonal bipyramidal geometry by asserting that electron pairs around a central atom will arrange themselves to minimize repulsion. In this case, five bonding pairs of electrons spread out into three equatorial and two axial positions to achieve the most stable arrangement. The bond angles of 120 degrees between equatorial positions and 90 degrees between axial and equatorial positions reflect this optimal spacing.
• Discuss how the presence of lone pairs can affect the geometry of a molecule originally predicted to be trigonal bipyramidal.
  • The presence of lone pairs can significantly alter the expected trigonal bipyramidal geometry. For instance, if one or more lone pairs occupy axial or equatorial positions, they will cause repulsion that distorts the shape. This can lead to geometries such as seesaw when one lone pair is present or T-shaped when two lone pairs occupy axial positions. These changes highlight how electron pair repulsion directly influences molecular shapes.
• Evaluate the implications of trigonal bipyramidal geometry on the properties and reactivity of compounds, using specific examples.
  • Trigonal bipyramidal geometry impacts both physical properties and chemical reactivity due to its spatial arrangement. For example, PCl5 shows high reactivity due to its ability to form different products depending on which bonds break during reactions. The axial and equatorial arrangements also affect dipole moments; molecules like SF6 have symmetrical arrangements leading to nonpolar characteristics, while asymmetric variants can exhibit polarity. Understanding these geometrical influences is crucial for predicting behavior in chemical reactions.

Related terms

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

Bond Angle: The angle formed between two bonds originating from the same atom, which is important in determining molecular shape.

Equatorial Position: The positions in a trigonal bipyramidal arrangement where three atoms lie in a plane around the central atom, typically having bond angles of 120 degrees.