VSEPR: Valence Shell Electron Pair Repulsion

Going back to the basics: The rule is that like charges repel each other. For example, electrons tend to repel each other, and if there were 2 electron pairs, those two pairs will also repel each other based on the rule stated above. Thus, within a molecule, electrons pairs will orient themselves such that there is maximum separation between them, which will lead to minimal repulsions. This desire to achieve minimum repulsions between electron pairs results in various shapes of molecules.

The VSEPR model was developed by Ronald J. Gillespie and R.S. Nyholm and is based on 3 main ideas:

  1. Chemical reactions and bonding involve only the electrons in the outermost or valence shell of an atom.
  2. Electrons in orbitals and in bonds are always in electrons pairs.
  3. Electrons repel one another because of like electrical charge.

Terminology:

Bonding pair (BP): 2 electrons involved in a bond

Lone pair (LP): 2 electrons NOT involved in a bond

Rules for using VSEPR theory: each pair of electrons in the valence shell occupies its own region of space called the domain of the electron pair.

  1. The bonding pairs (BP) and lone pairs (LP) move as far apart as possible to minimize electrostatic repulsions.
  2. A LP occupies slightly more volume than a BP
  3. Multiple bonds (double and triple) occupy more volume than a single bond.
  4. Polar bonds occupy less space at the central atom than non-polar bonds.

Note: VSEPR is only applicable to molecules or molecular ions. It does not apply to ionic compounds.

  1. The number of valence shell electron pairs determine shape:

Bonding pairs / Shape
2 pairs / Linear
3 pairs / Trigonal planar
4 pairs / Tetrahedral
5 pairs / Trigonal bipyramidal
6 pairs / Octahedral

Example 1: Silicon lies below carbon in group IV of the periodic table and is tetravalent. Predict the geometry of Silane, SiH4, which is a gas used in the preparation of extremely pure silicon for use in superconductors.

Example 2: Predict the shape of the hexachlorophosphate ion PCl6-1.

Example 3: Predict the geometry of chloroform CHCl3, which was at one time used as an anaesthetic.

  1. Lone electron pairs in the valence shell affect the shapes of molecules.

Example: Ammonia

The lone pair is more spread out – it is bulkier than a bonding pair of electrons and takes up more room than a covalent bond does. A lone pair also affects the bonding angles. It decreases the bond angles by 2 degrees.

Electron pair repulsion magnitudes decrease in the order:

LP – LP > LP – BP > BP –BP

  1. VSEPR theory is applicable to molecules with multiple bonds.

We count a double or triple bond as one group of electrons connecting the ligand X to the central atom A.

Carbon dioxide is classified as an AX2. Thus its molecular shape is linear.

What about HCN?

Predict the shape of formaldehyde: H2CO

Although, multiple bonds repel single bonds and thus multiple bonds act like lone pairs. Generally, we are going to treat a double or triple bond as a single electron pair and predict its shape using the rules applied for single bonded species.

  1. Lone pair electrons occupy the equatorial faces of trigonal bipyramidal molecule.

Placing the lone pair at an equatorial vertex rather than an axial vertex minimizes repulsion due to lone pairs.

AX4E AX3E2 AX2E3

  1. Two lone pairs occupy opposite vertices in an octahedron

AX6 AX5E AX4E2 AX3E3

To minimize the lone pair repulsion in AX4E2 the lone pairs are placed at opposite vertices.

Credits: J. Kennedy