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

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°


image of trigonal planar structure

 

 

 

 

Tetrahedral with angles of 109.5°


drawing of tetrahedron

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Trigonal planar molecular geometry (formaldehyde, CH2O)

ball and stick model of formaldehyde

ball and stick model of formaldehyde

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tetrahedral molecular geometry (methane, CH4)

ball and stick model of methane viewd in plane of HCH

ball and stick model of methane - offangle view

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

ball and stick model of ammonia - side view

rotated to view molecule from below

ball and stick model of ammonia - view from below

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bent molecular geometry (water, H2O)

ball and stick model of water

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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)

ball and stick model of phosphorus pentachloride

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

ball and stick model of sulfur tetrafluoride

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

T-shaped with angles of 90° (ClF3)

ball and stick model of Chlorine trifluoride

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Linear with angles of 180° (I3-)

ball and stick model of triodide ion

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Octahedral with angles of 90° (AsF6-)

ball and stick model of Arsenic hexafluoride ion

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tetragonal pyramidal with angles of 90° (ICl5)

ball and stick model of Iodine pentachloride

 

 

 

 

 

 

 

 

 

 

 

 

Square planar with angles of 90° (XeF4)

ball and stick model of Xenon tetrafluoride

 

© R A Paselk

Last modified 30 March 2011