Central Atom with One or More Lone Pairs

When people travel, they often take a lot more stuff than is needed. Trying to fit it all into a suitcase can be a real challenge. They may have to repack, or just squeeze it all in. Atoms often have to rearrange where their electrons are in order to create a more stable structure.

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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 onthe surrounding atoms (B) do not affect the geometry.

AB\(_3\)E: Ammonia, \(\ce{NH_3}\)

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

Figure \(\PageIndex{1}\): Lone pair electrons in ammonia. (CC BY-NC; CK-12)

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 \(\PageIndex{2}\): Ammonia molecule. (CC BY-NC; CK-12)

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 \(107^\text{o}\).

AB\(_2\)E\(_2\): Water, \(\ce{H_2O}\)

A water molecule consists of two bonding pairs and two lone pairs (see figure below).

Figure \(\PageIndex{3}\): Lone pair electrons on water.

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(CC BY-NC; CK-12)

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 \(\ce{H-O-H}\) bond angle is \(104.5^\text{o}\), which is smaller than the bond angle in \(\ce{NH_3}\) (see figure below).