Molecular Geometry: Axial and Equatorial Positions, Lone Pair Placement, and Practice Problems

Molecular Geometry & Bonding Theories

Axial and Equatorial Positions in Covalent Compounds

Covalent compounds with five or six electron groups exhibit distinct axial and equatorial positions for surrounding elements. Understanding these positions is crucial for predicting molecular shapes and electron pair arrangements.

• Equatorial Position: A surrounding element's position around the equator of a compound.

• Axial (Apical) Position: A surrounding element's position above or below the equatorial positions.

• These arrangements cause repulsion between elements and lone pairs for the compound.

• The most electronegative element tends to prefer the axial position over the equatorial position.

Example: Based on your knowledge of axial and equatorial positions, draw the most likely structure of PF5.

• PF5 has five electron groups, resulting in a trigonal bipyramidal geometry.

• Three positions are equatorial (in the plane), and two are axial (above and below the plane).

Diagram: (See original file for visual representation of axial and equatorial positions.)

Lone Pair Positions

Lone pairs will orient themselves to minimize repulsive interactions between surrounding elements. Their placement affects molecular geometry and stability.

• With 6 electron group systems, lone pairs are most stable in the axial position.

• With 5 electron group systems, lone pairs are most stable in the equatorial position.

Memory Tool: Use a clock analogy to remember the hands of the clock for electron group positions:

Electron Groups	Stable Lone Pair Position	Clock Analogy
5	Equatorial	3, 6, 9 o'clock
6	Axial	12, 6 o'clock

Practice Problems

Apply your understanding of molecular geometry and electron group positions to solve the following problems:

• Draw the most likely shape for the following compound: XeF4

  • XeF4 has six electron groups (four bonding pairs, two lone pairs).

  • Geometry: Square planar (lone pairs occupy axial positions).

• Draw and determine the geometry for the following molecule: Br2CO

  • Br2CO has three electron groups around the central carbon (two Br, one O).

  • Geometry: Trigonal planar.

• How many lone pairs reside in the equatorial position of the KCl32- ion?

  • Answer: 2 lone pairs (as indicated in the file).

Key Terms and Definitions

• Axial Position: Location above or below the equatorial plane in a molecule with trigonal bipyramidal or octahedral geometry.

• Equatorial Position: Location in the plane of the molecule, typically associated with lower repulsion for lone pairs in five electron group systems.

• Lone Pair: A pair of valence electrons not shared with another atom and thus not involved in bonding.

• Electron Group: Any lone pair, single bond, double bond, or triple bond around a central atom.

Relevant Equations

• Number of electron groups = Number of atoms bonded to central atom + Number of lone pairs on central atom

• For molecular geometry prediction, use VSEPR theory:

$\text{AX}_n\text{E}_m$

• Where A is the central atom, X is a bonded atom, n is the number of bonded atoms, and E is the number of lone pairs.

Examples and Applications

• PF5: Trigonal bipyramidal geometry, no lone pairs, all positions occupied by fluorine atoms.

• XeF4: Square planar geometry, two lone pairs occupy axial positions, four fluorine atoms in the plane.

• Br2CO: Trigonal planar geometry, no lone pairs on central carbon.

Additional info: The clock analogy is a mnemonic device to help students visualize electron group positions in molecular geometries, especially for trigonal bipyramidal and octahedral arrangements.