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Molecular Shapes: Molecules with Lone Pair(s) on Central Atom

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Molecular Shapes: Lone Pair(s) on Central Atom

How can all these clothes fit into such a small space?

When we travel, we often take a lot more stuff than we need.  Trying to fit it all in a suitcase can be a real challenge.  We may have to repack or just squeeze it all in.  Atoms often have to rearrange where the electrons are in order to create a more stable structure.

Central Atom with One or More Lone Pairs

The molecular geometries of molecules change when the central atom has one or more lone pairs of electrons.  The total number of electron pairs, both bonding pairs and lone pairs, leads to what is called the electron domain geometry.   When one or more of the bonding pairs of electrons is replaced with a lone pair, the molecular geometry (actual shape) of the molecule is altered.  In keeping with the A and B symbols established in the previous section, we will use E to represent a lone pair on the central atom (A).  A subscript will be used when there is more than one lone pair.  Lone pairs on the surrounding atoms (B) do not affect the geometry.

AB 3 E: Ammonia, NH 3

The ammonia molecule contains three single bonds and one lone pair on the central nitrogen atom.

Lone pair electrons in ammonia.

The domain geometry for a molecule with four electron pairs is tetrahedral, as was seen with CH 4 .  In the ammonia molecule, one of the electron pairs is a lone pair rather than a bonding pair.  The molecular geometry of NH 3 is called trigonal pyramidal.

Ammonia molecule.

Recall that the bond angle in the tetrahedral CH 4 molecule is 109.5°.  Again, the replacement of one of the bonded electron pairs with a lone pair compresses the angle slightly.  The H-N-H angle is approximately 107°.

AB 2 E 2 : Water, H 2 O

A water molecule consists of two bonding pairs and two lone pairs.

Lone pair electrons on water.

As for methane and ammonia, the domain geometry for a molecule with four electron pairs is tetrahedral.  In the water molecule, two of the electron pairs are lone pairs rather than bonding pairs.  The molecular geometry of the water molecule is bent.  The H-O-H bond angle is 104.5°, which is smaller than the bond angle in NH 3 .

Water molecule.

AB 4 E: Sulfur Tetrafluoride, SF 4

The Lewis structure for SF 4 contains four single bonds and a lone pair on the sulfur atom.

Lone pair electrons in SF 4 .

The sulfur atom has five electron groups around it, which corresponds to the trigonal bipyramidal domain geometry, as in PCl 5 .  Recall that the trigonal bipyramidal geometry has three equatorial atoms and two axial atoms attached to the central atom.  Because of the greater repulsion of a lone pair, it is one of the equatorial atoms that are replaced by a lone pair.  The geometry of the molecule is called a distorted tetrahedron or seesaw.

Ball and stick model for SF 4 .

Geometries in Which the Central Atom Has One or More Lone Pairs
Total Number of Electron Pairs Number of Bonding Pairs Number of Lone Pairs Electron Domain Geometry Molecular Geometry Examples
3 2 1 trigonal planar bent O 3
4 3 1 tetrahedral trigonal pyramidal NH 3
4 2 2 tetrahedral bent H 2 O
5 4 1 trigonal bipyramidal distorted tetrahedron (seesaw) SF 4
5 3 2 trigonal bipyramidal T-shaped CIF 3
5 2 3 trigonal bipyramidal linear I 3 -
6 5 1 octahedral square pyramidal BrF 5
6 4 2 octahedral square planar XeF 4

Summary

  • The presence of lone pair electrons influences the three-dimensional shape of the molecule.

Practice

Use the link below to answer the following questions:

  1. What is the general principle in dealing with molecules containing more than four electron pairs?
  2. In the picture with five electron pairs around the central atom, why is the arrangement on the right preferred?
  3. In the picture with six electron pairs, why is the configuration with the lone pairs at 180 o to each other more stable?

Review

  1. Why does water have a bent geometry?
  2. Why is ammonia not a planar molecule?
  3. How would we write the configuration for xenon tetrafluoride using the ABE system?

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