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 = "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°


image of trigonal planar structure

 

 

 

 

Tetrahedral with angles of 109.5°


drawing of tetrahedron

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CO2 linear molecular geometry

ball and stick model of carbon dioxide

 

 

 

 

 

 

 

 

 

 

 

Trigonal planar molecular geometry (formaldehyde, CH2O)

Lewis Structure for formaldehyde

ball and stick model of formaldehyde

ball and stick model of formaldehyde

 

 

 

 

 

 

 

 

 

 

 

Tetrahedral molecular geometry (methane, CH4)

Lewis Structure for methane

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]

Lewis Structure for ammonia

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)

Lewis Structure for water

ball and stick model of water

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Trigonal planar with angles of 120°


image of trigonal planar structure

 

 

 

 

Tetrahedral with angles of 109.5°


drawing of tetrahedron

 

 

 

 

CO2 linear molecular geometry

ball and stick model of carbon dioxide

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, carbon dioxide dipolesso the molecule is not polar.

 

 

 

 

 

 

Trigonal planar molecular geometry (formaldehyde, CH2O)

ball and stick model of formaldehyde

ball and stick model of formaldehyde

 

 

 

 

 

 

 

 

 

 

 

 

 

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:

ball and stick model of formaldehydewith dipole

  • 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)

drawing of tetrahedron

ball and stick model of methane viewd in plane of HCH

ball and stick model of methane - offangle view

 

 

 

 

 

 

 

 

 

 

 

 

 

Non-polar

 

 

 

 

 

drawing of tetrahedron

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

    • only 3 atoms so trigonal pyramidal molecular geometry

    ball and stick model of ammonia rotated to a different angle = ball and stick model of ammonia

 

 

 

 

 

 

 

 

 

 

 

 

 

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:

ball and stick model of ammonia with dipole arrow

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

 

 

 

 

 

drawing of tetrahedron

Bent molecular geometry (water, H2O)

ball and stick model of water rotated = ball and stick model of water

 

 

 

 

 

 

 

 

 

 

 

 

 

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. dipoles of H-O bonds of water
  • The two dipoles add together in the water molecule which is polar as shown with the dipole arrow in the image: Water dipole

ball and stick model of water with dipolee

 

© R A Paselk

Last modified 21 October 2009