VSEPR (Valence Shell Electron Pair Repulsion) Theory is based on three assumptions (there are more advanced versions, but unnecessary for us):

• Electron pairs will orient around a central point to minimize repulsion.

• Lone-pairs of electrons will have greater repulsion than bonded pairs of electrons (note that the atoms are ignored in terms of repulsion).

• Repulsion is strong at 90° and weaker at 120° (weakest at 180°).

VSEPR predicts geometry based on these assumptions in a few simple, sequential, steps:

1. Draw a correct Lewis Structure.

2. Determine the Steric Number = the number of bonded atoms + the number of lone pairs.

3. Maximize the angles between electron pairs, placing the lone (unbonded) pairs at the extremes.

Trigonal planar with angles of 120°

Tetrahedral with angles of 109.5°

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Trigonal planar molecular geometry (formaldehyde, CH₂O)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tetrahedral molecular geometry (methane, CH₄)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Trigonal pyramidal molecular geometry (ammonia, NH₃) [model]

rotated to view molecule from below

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bent molecular geometry (water, H₂O)

 

Exceptions to the "Octet Rule."

To help determine if the octet rule is followed recall Clark's Method (abbreviated) for determining bonding in covalent Lewis Structures: Add up all of the valence electrons in the structure (remember to add one electron for each negative charge, or subtract one for each positive charge) If e- = 6y + 2 where y = # atoms other than H, then octet rule is followed with single bonds only. If e- < 6y + 2 then probably have multiple bonding with the number of multiple bonds = /2 (remember a triple bond is 2 multiple bonds!). However, note the exceptions below with small atoms (H, Li, Be, and B). If e- > 6y + 2 then have an expanded valence shell. Note that if = 2, then pentavalent (10 electrons in the valence shell) , and if = 4, then hexavalent (12 electrons in the valence shell). If you can draw more than one structure, then chose the most symmetrical. If two or more structures are equally symmetrical, then you probably have resonance and should show all structures connected by double arrows.

 

Trigonal bipyramidal with angles of 90° & 120° (PCl₅)

Seesaw with angles of 90° & 120° (SF₄)

T-shaped with angles of 90° (ClF₃)

Linear with angles of 180° (I₃^-)

Octahedral with angles of 90° (AsF₆^-)

Tetragonal pyramidal with angles of 90° (ICl₅)

Square planar with angles of 90° (XeF₄)

© R A Paselk

Last modified 12 April2013