shapes of molecules finding the lone pairs

Shapes of trigonal bypramidal molecules with lone pairs

Trigonal bipyramidal (tbp) molecules can also be found with lone pairs. This creates a bit of a dilemma, since there are two differential positions available in these molecules, that is the axial and the equatorial positions. So if a tbp molecule has a lone pair of electrons, will it go into the axial or the equatorial position? Now remember that the lone pair requires more space than the bonding pair, so where in a trigonal bipyramidal molecule will the lone pair have most space? In the axial or equatorial positions?

In the axial position the lone pair will experience more repulsion since the 3 bonding pairs in the equatorial position are only 900 away whereas in the equatorial position there are two bonding pairs at 900 and the other equatorial bonding pairs are much further away. This means in a trigonal bipyramidal molecule the lone pairs always occupy the equatorial position.

Example - what shape is sulfur tetrafluoride (SF4)

sulfur tetrafluoride is see-saw shaped Using the VSEPR we have used before we get:

  1. Sulfur is the central atom and it is in group 6. It has 6e valency electrons
  2. Four fluorine are bonded to the central atom, each contribute 1 electron. So we have 4 electrons in total.
  3. The total number of electrons in the valency shells is 10 electrons, dividing by 2 gives 5 electron pairs, so the shape of the SF4 molecule will be based on a trigonal bipyramidal shape. However the trigonal bipyramidal structure has 5 atoms around the central one, but in SF4 there are only 4 fluorine atoms around the central sulfur atom. This means that there is one lone pair in this molecule and it will occupy the equatorial position.

Remember to get the overall shape we need to imagine the structure without the lone pairs. This is shown below:

 shape of this molecule is described as see-saw

Example 2- What shaped is chlorine trifluoride (ClF3)?

  1. Chlorine is the central atom and it is in group7. It has 7e valency electrons
  2. Three fluorine are bonded to the central atom, each contribute 1 electron. So we have 3 electrons in total.
  3. The total number of electrons in the valency shells is 10 electrons, dividing by 2 gives 5 electron pairs, so the shape of the ClF3 molecule will be based on a trigonal bipyramidal shape. There will be 2 lone pairs (4 electrons in total) and 3 bonding pairs, one pair for each of the fluorine chlorine bonds. Without the lone pairs present the molecule is simply described as T-shaped for obvious reasons!

shape of chlorine trifluoride is T-shaped

Example 3- What shaped is xenon difluoride (XeF2)?

The noble gases are I am sure you are aware are generally very unreactive, however the larger noble gas elements such as xenon can be made to react with elements such as fluorine, xenon difluoride is a powerful fluorinating agent with the chemical formula XeF2. What shape will this molecule have? Simply use the VSEPR rules we have used so far to predict its shape:

  1. Xenon is the central atom and it is in group8. It has 8e valency electrons
  2. Two fluorine are bonded to the central atom, each contribute 1 electron. So we have 2 electrons in total.
  3. The total number of electrons in the valency shells is 10 electrons, dividing by 2 gives 5 electron pairs, so the shape of the XeF2 molecule will be based on a trigonal bipyramidal shape. There will be 3 lone pairs (6 electrons in total) and 2 bonding pairs, one pair for each of the xenon fluorine bonds.
    The 3 lone pairs will occupy the equatorial positions with bond angles of 1200 and the two fluorine atoms will in both axial positions. The overall shape of the molecule if we ignore the lone pairs is simply linear. The molecule is perfectly straight!

Key Points


Practice questions

Check your understanding - Questions on shapes and lone pairs

Next