What shapes are associated with five electron pairs?

Short Answer

When a central atom has five electron pairs, the electron-pair geometry is trigonal bipyramidal. These five electron pairs arrange themselves to minimise repulsion, creating three positions in a plane (equatorial) and two positions above and below the plane (axial).

However, the molecular shape depends on how many of these five electron pairs are bonding pairs and how many are lone pairs. The shapes associated with five electron pairs include trigonal bipyramidal, seesaw, T-shaped, and linear.

Detailed Explanation :

Shapes Associated with Five Electron Pairs

When a molecule has five electron pairs around the central atom, VSEPR theory states that these electron pairs arrange themselves in a trigonal bipyramidal electron geometry. This arrangement leads to three positions in a horizontal plane (equatorial positions) and two positions vertical to the plane (axial positions).

The electron geometry remains the same (trigonal bipyramidal), but the molecular shape changes depending on how many electron pairs are bonding pairs (forming bonds) and how many are lone pairs (non-bonding).

Because lone pairs repel more strongly, they occupy equatorial positions first, causing distortions in shape. As a result, five electron pairs can lead to four important molecular shapes: trigonal bipyramidal, seesaw, T-shaped, and linear.

  1. Trigonal Bipyramidal (AX₅)

This is the ideal shape when all five electron pairs are bonding pairs, meaning there are no lone pairs on the central atom.

Geometry Features

  • Three equatorial bonds at 120°
  • Two axial bonds at 90°

Why this shape forms

With no lone pairs, repulsion is symmetrical, so all five bonds spread out evenly.

Examples

  • Phosphorus pentachloride (PCl₅)
  • PF₅

In this shape, the molecule is perfectly trigonal bipyramidal.

  1. Seesaw Shape (AX₄E)

When one of the five electron pairs is a lone pair, the shape becomes seesaw.

Why seesaw forms

  • Lone pair takes one equatorial position to minimise repulsion.
  • Remaining four bonding pairs arrange asymmetrically, giving a seesaw structure.

Bond Angle Effects

  • Equatorial bond angles shrink slightly (< 120°).
  • Axial bonds become slightly longer (< 90°).

Examples

  • Sulfur tetrafluoride (SF₄)

This shape reflects the imbalance created by one lone pair on a trigonal bipyramidal electron arrangement.

  1. T-Shaped Shape (AX₃E₂)

When two of the five electron pairs are lone pairs, they both occupy equatorial positions. The remaining three bonding pairs create a T-shaped structure.

Why it forms

  • Lone pairs repel strongly and stay 120° apart.
  • Three bonded atoms stay mostly in a straight line with one bent bond forming the “T”.

Examples

  • Chlorine trifluoride (ClF₃)
  • BrF₃

This shape is highly distorted because of strong lone pair repulsion.

  1. Linear Shape (AX₂E₃)

When three of the five electron pairs are lone pairs, the molecule becomes linear.

Why linear forms

  • Three lone pairs occupy all equatorial positions, which maximises their separation.
  • Two axial positions remain for the bonding pairs.
  • Axial positions are exactly 180° apart, producing a linear shape.

Examples

  • Xenon difluoride (XeF₂)

Even though electron geometry is trigonal bipyramidal, the molecular shape is linear because atoms lie only along the axial line.

  1. Why Different Shapes Form from the Same Electron Geometry

The differences in shape arise from:

  • Strength of lone pair repulsion
  • Preference of lone pairs for equatorial positions
  • Distortion of bond angles due to non-bonding electron pressure

Lone pairs always cause:

  • Greater repulsion
  • Smaller bond angles
  • More distortion

Thus, one electron geometry (trigonal bipyramidal) gives rise to several molecular shapes depending on the number of lone pairs.

  1. Summary of Shapes Based on Lone Pairs
Electron Pair Arrangement Molecular Shape
AX₅ Trigonal bipyramidal
AX₄E Seesaw
AX₃E₂ T-shaped
AX₂E₃ Linear

This shows how adding lone pairs changes the final shape even though the electron geometry stays the same.

Conclusion

Five electron pairs around a central atom result in a trigonal bipyramidal electron geometry. Depending on how many of these pairs are lone pairs, several molecular shapes can form: trigonal bipyramidal (no lone pair), seesaw (one lone pair), T-shaped (two lone pairs), and linear (three lone pairs). These variations arise because lone pairs repel more strongly and occupy equatorial positions, causing changes in molecular geometry.