ELECTRON DOMAINS AND MOLECULAR SHAPE



The electronic structure of a molecule or ion provides a wonderful avenue through which to explore the resulting shape of the chemical entity. Electron domains (or groups) are classified by the number of spaces you would most probably find electrons around a central atom. The AXnEm classification system uses A to denote the central atom, X to denote the number of bonding electron pairs, and E to denote the lone electron pairs around the central atom. Subscripts on X and E indicate the number of each present.
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Linear (AX2)

This is an example of the two electron domain system (linear domain), and the linear molecular shape. Linear molecules can consist of several atoms arranged in a line. Around each of the atoms in the structure are only two electron domains (groups). These electron groups are arranged 180 degrees apart. Consequently, the common thread through all of these molecules of this type is the 180 degree angles formed by all of the bonds. For example, allene, CH2=C=CH2, consists of two H2C-C bonds of length 1.31 Angstroms and form a 180 degree angle. Carbon dioxide, CO2, is also a linear species. It consists of two O-C bonds of length 1.16 Angstroms that form a 180 degree angle.

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Trigonal Planar (AX3)

This is an example of a three electron domain system (trigonal planar domain) and a trigonal planar molecule. These molecules have four atoms in a triangular arrangement. All angles add up to 360 degrees. For example, boron trifluoride (BF3), has 3 F-B bonds, all of length 1.30 Angstroms, which form 120 degree angles. Carbonyl fluoride, or CF2O, has two F-C bonds and one O=C bond. The F-C bonds are 1.31 Angstroms, and the O=C bond is 1.17 Angstroms. The angles between the O=C bond and an individual F-C bond are 126 degrees. The angle between the two F-C bonds is 108 degrees. This illustrates how an double bond takes up more space than a single bond.

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Bent (AX2E, AX2E2)

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This is an example of a bent molecule. Bent molecular shapes can arise from two different electron domain configurations, the AX2E (trigonal planar) or AX2E2 (tetrahedral). The difference between the two is that AX2E2 has one more lone pair around the central atom. The effect of these lone pairs on a molecule is significant. Lone pairs occupy s orbitals, leaving the bonds or shared electrons in the p orbitals. P orbitals are naturally more perpendicular. The result is a smaller angle. A good demonstration of this effect is with NO2 and NO2+. NO2 is a bent molecule; however, when you remove an electron from it, making it NO2+, the molecule becomes linear due to the loss of a lone electron. In NO2+, there is no repulsion taking place between the two O atoms and the lone electron on the central atom. Sulfur difluoride, SF2, is an AX2E2 species, and it has an angle of 98 degrees. On the other hand, nitrogen dioxide, NO2, is an AX2E species, and it has an angle of 134 degrees. The additional lone pair on the SF2 molecule makes the angle smaller.

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Tetrahedral (AX4)

This is an example of a four electron domain system (tetrahedral domain) and a tetrahedral molecule. These molecules consist of 4 atoms surrounding a central atom, creating four faces.
Tetrahedral molecules are under the category, AX4 and have sp3 sigma hybridization. Methane, or CH4, is a good example of a tetrahedral molecule. It has four C-H bonds with no lone pairs on any atom. It is described as a perfect tetrahedron because all of its angles are 109.45 degrees and the bonds are all 1.09 Angstroms. Carbon tetrafluoride, or CF4 is also a perfect tetrahedral molecule. It has four C-F bonds, with C as the central atom. The four F atoms have 3 lone pairs on each. The length of the F-C bonds are all 1.32 Angstroms forming equal 109.45 degree angles. Sulfuric acid or H2SO4 is also a tetrahedral molecule. Sulfuric acid has sulfur as its central atom, and has two O atoms, and two OH groups surrounding it. The HO-S-OH angle is smaller than the O=S=O angle. This is because the double bonds between O and S create a shorter distance and the OH groups are repelled by the O atoms coming closer. As a result, the O=S=O angle is larger than the HO-S-OH angle.

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Trigonal Pyramidal (AX3E)

This is an example of a trigonal pyramidal molecule in a tetrahedral electron domain. These molecules have four atoms in the shape of a pyramid with a base in the shape of a triangle. AX3E signifies that the molecule has a lone electron pair around the central atom. This lone pair is pushed away from the adjacent atoms and exists around the central atom. An example of a trigonal pyramidal molecule is ammonia, NH3, which has a nitrogen atom as its central atom, and three hydrogens surrounding it, forming the "base of the pyramid." In this case, the angles are equal to each other, but because it is not a planar molecule, the angles will not add up to 360 degrees. Some trigonal pyramidal molecules will not have a symmetrical base because the angles will not all be the same. For example, methylamine, NH2CH3, has a nitrogen as its central atom, and is surrounded by two hydrogen atoms and a CH3 group. This molecule does not have a symmetrical base. Its angles are 112.2, 112.2, and 105.8 degrees respectively. Also rememeber than lone electron pairs take up more space than do bonding pair electrons.

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Trigonal Bipyramidal (AX5)

This is an example of a five electron domain system (trigonal bipyramidal domain) and a trigonal bipyramidal molecule. There is a central atom surrounded by 5 adjacent atoms. Three of these adjacent atoms exist on the equatorial plane, and 2 are on the axial plane. Denote the atoms on the equatorial plane X1, the atoms on the axial plane X2, and the central atom as A. In a perfect trigonal bypyramidal molecule, the angles formed by X1-A-X1, are 120 degrees; the angles formed by X2-A-X1 are 90 degrees. For example, in phosphorus pentafluoride, PF5, the F1-P-F2 angle is 90 degrees, and the F1-P-F1 angle is 120 degrees. Not all trigonal bypyramidal molecules are this ideal. For example, sulfur oxide tetrafluoride, SOF4, has a F1-S-O angle of 125 degrees, and a F1-S-F1 angle of 110.












Seesaw Shaped (AX4E)

This is an example of a seesaw shaped molecule within a trigonal bipyramidal electron domain. These molecules are much less common than trigonal pyramidal and tetrahedral molecules. The central atom is surrounded by four adjacent atoms, two on the same plane (axial) and two below (equatorial). This shape is caused by a lone pair of electrons on the central atom. An example of a seesaw shaped molecule is sulfur tetrafluoride, or SF4. Sulfur is the central atom, two fluorine atoms are on the equatorial plane, and two are on the axial plane. The angle formed by the F-S-F axial plane is 173 degrees and 3.28 Angstroms in length, which can be attributed to the lone pair of electrons on the S atom. The equatorial plane fluorine atoms form an angle of 89 degrees, and the angle formed from the equatorial plane fluorine atoms to the axial plane fluorine atoms is 103 degrees.

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T-Shaped (AX3E2)

This is an example of a t-shaped molecule in a trigonal bipyramidal electron domain. These molecules consist of three atoms surrounding a central atom. The central atom has two lone pairs of electrons which make the angles close to 90 degrees. The lone pairs on the central atom repel the adjacent atoms, preventing the molecule from taking the shape of a trigonal planar molecule. An example of a t-shaped molecule is chlorine trifluoride, ClF3. Call the axial atoms F2, and the basal atom F1. The F2-Cl-F1 angle is 87.5 degrees, and the F2-Cl bond length is 1.70 Angstroms. The F1-Cl bond length is 1.60 Angstroms. These angles will vary as the outer atoms change in nucleic strength from molecule to molecule.

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Octahedral (AX6)

This is an example of a six electron domain (octahedral) system and an octahedral molecule. These molecules have 6 atoms surrounding a central atom. The atoms directly across from each other are called "trans," and the atoms next to each other are called "cis." The ideal octahedron has 90 degree angles between bonds and bond lengths of equal distances. Sulfur hexafluoride is a commonly used example because it has all 90 degree angles and equal distances of 1.56 Angstroms on the F-S bonds.

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Square Pyramidal (AX5E)

This is an example of a square pyramidal molecule within an octahedral electron domain. There are five atoms surrounding a central atom with a lone pair of electrons. The outer atom that is at the very top of the molecule is in the "apical position." The four surrounding atoms that are at the base of the molecule are known as the "basal atoms," and the lone pair exists on the central atom. The apical A-X bond distance is always shorter than the basal-central bond distances. The apical-center-basal angle is much different from molecule to molecule. In the molecule BF5, the central atom is actually lower than the basal plane, so the square pyramidal molecule does not always have the exact shape shown to the left. In the case of BF5, the reason that the central atom is pushed down is because of electron repulsion from the lone pair.

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Square Planar (AX4E2)

This is an example of a square planar molecule within an octahedral electron domain system. It consists of 4 atoms surrounding a central atom. The structure is in the shape of a square on a plane. All four angles will add up to 360 degrees, but do not necessarily have to be 90 degrees each. An example of a square planar molecule is xenon tetrafluoride, XeF4. It has four Xe-F bonds, 1.95 Angstroms in length, which form four 90 degree angles. It is a perfect square planar molecule.












Geometry Summary Table


Total Domains e- Domain Geometry Bonding Domains Nonbonding Domains Molecular Geometry
2 Linear 2 0 Linear
3 Trigonal Planar 3 0 Trigonal Planar
3 Trigonal Planar 2 1 Bent
4 Tetrahedral 4 0 Tetrahedral
4 Tetrahedral 3 1 Trigonal Pyramidal
4 Tetrahedral 2 2 Bent
5 Trigonal Bipyramidal 5 0 Trigonal Bipyramidal
5 Trigonal Bipyramidal 4 1 Seesaw
5 Trigonal Bipyramidal 3 2 T-shaped
5 Trigonal Bipyramidal 2 3 Linear
6 Octahedral 6 0 Octahedral
6 Octahedral 5 1 Square Pyramidal
6 Octahedral 4 2 Square Planar

Modified and Updated 25 October 2000, PTJ

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