VSEPR Theory

Valence shell electron-pair repulsion theory (VSEPR theory) enables us to predict the molecular structure, including approximate bond angles around a central atom, of a molecule from an examination of the number of bonds (bonding domains) and lone electron pairs (non-bonding domains) in its Lewis structure.

The VSEPR model assumes that electron pairs in the valence shell of a central atom will adopt an arrangement that minimizes electrostatic repulsions between these electron pairs by maximizing the distance between them.

A drawback of this theory is that it only considers electron-pair repulsions. Other interactions, such as nuclear-nuclear repulsions and nuclear-electron attractions, are also involved in the final arrangement that atoms adopt in a particular molecular structure.

A lot of new terminology was just introduced, some of the terminology is the same and some is different from what you have experienced with drawing Lewis structures previously. Let’s look at the terminology of Lewis structures first with the example of ammonia, NH3:

See how some of these terms are the same or different when now looking at the VSEPR structure for the same molecule of ammonia, NH3:

A lot of new terminology has been defined in the image above. Here is a list of them for reference:

  • bonding domain – a set of electrons involved in bonding between two atoms. A single, double or triple bond all count as one single bonding domain as they are all bonding out to a single atom.
  • non-bonding domain – a lone pair, pair of electrons that only belong to a single atom and are not involved in bonding (in the rare case of a single electron, a radical, the are treated as a non-bonding domain).
  • bond angle – is the angle between two bonding domains and helps define the geometry/shape.
  • dash/hash line – a bonding domain going into the page (fading away from the viewer).
  • wedge line – a bonding domain coming out of the page (coming at the viewer).
  • bond in line with page – a bonding domain that is in line witn the page (in the plane of the paper).
  • central atom – an atom where we are defining geometry/shape for. In a species with multiple central atoms we need to determine the geometry/shape for each central atom.
  • terminal atom – an atom off of the central atom that we are determining the shape for.

Use your understanding of these definitions and the labeled diagram above for NH3 to help you properly label the VSEPR of Methane, CH4: