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 = "electron clouds" in valance shell of central atom.

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°

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CO₂ linear molecular geometry

 

 

 

 

 

 

 

 

 

 

 

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)

 

Trigonal planar with angles of 120°

 

 

 

 

Tetrahedral with angles of 109.5°

 

 

 

 

CO₂ linear molecular geometry

Polarity:

• C and O have significantly different Electronegativity values (2.5 & 3.5), so the C-O bond will be polar, with O partially negative.
• However, the two polar bonds exactly cancel each other since they point in opposite directions, so the molecule is not polar.

 

 

 

 

 

 

Trigonal planar molecular geometry (formaldehyde, CH₂O)

 

 

 

 

 

 

 

 

 

 

 

 

 

Polarity:

• C and O have significantly different Electronegativity values (2.5 & 3.5), so the C-O bond will be polar, with O partially negative. C and H differ only slightly in EN (2.5 & 2.1), so the C-H bonds will be only slightly polar.
• Formaldehyde is polar as shown with the dipole arrow in the image below:

• The polar contributions of the angled slightly polar C-H bonds will not cancel the C-O bond, so overall the molecule is polar.

 

 

 

 

 

Tetrahedral Electronic geometry

Tetrahedral molecular geometry (methane, CH₄)

 

 

 

 

 

 

 

 

 

 

 

 

 

Non-polar

 

 

 

 

 

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

• only 3 atoms so trigonal pyramidal molecular geometry

rotated to a different angle =

 

 

 

 

 

 

 

 

 

 

 

 

 

Polarity:

• H and N have significantly different Electronegativity values (2.1& 3.0), so the H-N bond will be polar, with N partially negative.
• Ammonia is polar as shown with the dipole arrow in the image below:

• The polar contributions of the angled polar H-O bonds along the central axis add up so overall the molecule is polar.

 

 

 

 

 

Bent molecular geometry (water, H₂O)

rotated =

 

 

 

 

 

 

 

 

 

 

 

 

 

Polarity:

• H and O have significantly different Electronegativity values (2.1& 3.5), so the H-O bond will be polar, with O partially negative.
• The two dipoles add together in the water molecule which is polar as shown with the dipole arrow in the image:

 

© R A Paselk

Last modified 21 October 2009