What is the molecular geometry of a molecule with 2 outer atoms and 1 lone pair on the central atom?

The molecular geometry of a molecule can be determined using the VSEPR theory. VSEPR (Valence Shell Electron Pair Repulsion) Theory: The basic premise of this simple theory is that electron pairs (bonding and nonbonding) repel one another; so the electron pairs will adopt a geometry about an atom that minimizes these repulsions. Use the method below to determine the molecular geometry about an atom. Write the Lewis dot structure for the molecule. Count the number of things (atoms, groups of atoms, and lone pairs of electrons) that are directly attached to the central atom (the atom of interest) to determine the overall (electronic) geometry of the molecule. Now ignore the lone pairs of electrons to get the molecular geometry of the molecule. The molecular geometry describes the arrangement of the atoms only and not the lone pairs of electrons. If there are no lone pairs in the molecule, then the overall geometry and the molecular geometry are the same. If the overall geometry is tetrahedral, then there are three possibilities for the molecular geometry; if it is trigonal planar, there are two possibilities; and if it is linear, the molecular geometry must also be linear. The diagram below illustrates the relationship between overall (electronic) and molecular geometries. To view the geometry in greater detail, simply click on that geometry in the graphic below. Although there are many, many different geometries that molecules adopt, we are only concerned with the five shown below.

Figure \(\PageIndex{1}\) (Credit: Image copyright Jandrie Lombard, 2014; Source: http://www.shutterstock.com(opens in new window); License: Used under license from Shutterstock.com)

When we travel, we often take a lot more stuff than we need. Trying to fit it all into 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.

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.

The ammonia molecule contains three single bonds and one lone pair on the central nitrogen atom (see figure below).

Figure \(\PageIndex{2}\): Lone pair electrons in ammonia. (Credit: Joy Sheng; Source: CK-12 Foundation; License: CC BY-NC 3.0(opens in new window))

The domain geometry for a molecule with four electron pairs is tetrahedral, as was seen with \(\ce{CH_4}\). In the ammonia molecule, one of the electron pairs is a lone pair rather than a bonding pair. The molecular geometry of \(\ce{NH_3}\) is called trigonal pyramidal (see figure below).

Figure \(\PageIndex{3}\): Ammonia molecule. (Credit: (Left) User: Booyabazooka/ Wikimedia Commons; (Right) Ben Mills (Wikimedia: Benjah-bmm27); Source: (Left) http://commons.wikimedia.org/wiki/File:Ammonia_lone_electron_pair.svg(opens in new window); (Right) http://commons.wikimedia.org/wiki/File:Ammonia-3D-balls-A.png(opens in new window); License: Public Domain)

Recall that the bond angle in the tetrahedral \(\ce{CH_4}\) molecule is \(109.5^\text{o}\). Again, the replacement of one of the bonded electron pairs with a lone pair compresses the angle slightly. The \(\ce{H-N-H}\) angle is approximately \(107^\text{o}\).

The Lewis structure for \(\ce{SF_4}\) contains four single bonds and a lone pair on the sulfur atom (see figure below).

Figure \(\PageIndex{6}\): Lone pair electrons in \(\ce{SF_4}\). (Credit: Joy Sheng; Source: CK-12 Foundation; License: CC BY-NC 3.0(opens in new window))

The sulfur atom has five electron groups around it, which corresponds to the trigonal bipyramidal domain geometry, as in \(\ce{PCl_5}\) (see figure below). 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.

Figure \(\PageIndex{7}\): Ball and stick model for \(\ce{SF_4}\). (Credit: Ben Mills (Wikimedia: Benjah-bmm27); Source: http://commons.wikimedia.org/wiki/File:Sulfur-tetrafluoride-3D-balls.png(opens in new window); License: Public Domain) Table \(\PageIndex{1}\): 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	\(\ce{O_3}\)
4	3	1	Tetrahedral	Trigonal Pyramidal	\(\ce{NH_3}\)
4	2	2	Tetrahedral	Bent	\(\ce{H_2O}\)
5	4	1	Trigonal Bipyramidal	Distorted Tetrahedron (Seesaw)	\(\ce{SF_4}\)
5	3	2	Trigonal Bipyramidal	T-shaped	\(\ce{ClF_3}\)
5	2	3	Trigonal Bipyramidal	Linear	\(\ce{I_3^-}\)
6	5	1	Octahedral	Square Pyramidal	\(\ce{BrF_5}\)
6	4	2	Octahedral	Square Planar	\(\ce{XeF_4}\)

Summary

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

Review

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

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