shapes of molecules finding the lone pairs

Before reading this page make sure you know how to work out the basic shapes of molecules using the VSEPR model and you know how to find lone pairs in simple molecules.

Shapes of trigonal bipyramidal 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 they go into the axial or the equatorial position? Now remember that a lone pair of electrons requires more space than a bonding pair of electrons; so where in a trigonal bipyramidal molecule will the lone pair of electrons have the most space? In the axial or equatorial positions?

Trigonal bipyramidal molecules have more space in the equatorial positions than in the axial positions for lone pairs of electrons.  There is less repulsion in the axial positions.

In the axial position the lone pair of electrons will experience more repulsion since the three bonding pairs of electrons 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 that in a trigonal bipyramidal molecule if there are lone pairs of electrons present they will occupy the equatorial positions.

Example - what shape is sulfur tetrafluoride (SF4)

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

  1. Sulfur is the central atom and it is in group 6 of the periodic table. It has means it has 6 valency electrons it its outer shell.
  2. Four fluorine atoms are bonded to the central sulfur atom and each fluorine atom will contribute 1 electron to the covalent bond with the central sulfur atom. So we have 4 electrons in total from the fluorine atoms.
  3. The total number of electrons in the valency shells is therefore 10 electrons and dividing by 2 since each covalent bond involves the sharing of a pair of electrons 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 bonded to the central atom but in SF4 there are only 4 fluorine atoms around the central sulfur atom. This means that there is one lone pair or non-bonding of electrons in this molecule and it will occupy the equatorial position where there is more space for it.

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

 sulfur htetrafluoride (SF4) is described as having a  see-saw shape.

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

  1. Chlorine is the central atom and it is found in group7 of the periodic table; so it has 7 valency electrons.
  2. Three fluorine atoms are bonded to the central atom chlorine atom with each fluorine atom contributing 1 electron to each covalent bond to the chlorine atom. So we have 3 electrons in total.
  3. The total number of electrons in the valency shells is therefore 10 electrons; dividing by two 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 of electrons (4 electrons in total) and 3 bonding pairs of electrons (6 electrons in total); one pair for each of the fluorine chlorine bonds. The shape of this molecule is shown below; without the lone pairs of electrons 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)?

shape of XeF2 is linear The noble gases are generally very unreactive; however the larger noble gas such as xenon will react with fluorine to form xenon difluoride (XeF2). What shape will this molecule have? Well as before simply use the VSEPR rules we have used so far to work out its shape:

  1. Xenon is the central atom and it is in group8 of the periodic table. It has 8 valency electrons.
  2. Two fluorine atoms are bonded to the central xenon atom and each fluorine atom will contribute 1 electron to the covalent bond with the central xenon atom. So we have 2 electrons in total from the fluorine atoms.
  3. The total number of electrons in the valency shells is therefore 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 of electrons (6 electrons in total) and 2 bonding pairs of electrons; 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 ignoring the lone pairs of electrons is simply linear. The molecule is perfectly straight!

Key Points


Practice questions

Check your understanding - Questions on shapes and lone pairs

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