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AXE method

In chemistry, the AXE method is commonly used in formatting molecules to fit the VSEPR model that aims to explain molecular geometry.

The A represents the central atom and always has an implied subscript one. The X represents how many sigma bonds are formed between the central atoms and outside atoms. Multiple covalent bonds (double, triple, etc) count as one X. The E represents the number of lone electron pairs present outside of the central atom. The sum of X and E represents the total number of hybridised orbitals (sometimes known as the steric number), which determines the type of hybridisation undergone in the central atom (2 = sp, 3 = sp2, 4 = sp3, 5 = sp3d, 6 = sp3d2, 7 = ?). Once the AXE formula has been found, the following table will predict the geometric configuration around the central atom:


Type Shape Geometry Geometry Examples
AX1E* Linear (N/A) HF, O2
AX2E0 Linear BeCl2, HgCl2, CO2
AX2E1 Bent NO2, SO2, O3
AX2E2 Bent H2O, OF2
AX2E3 Linear XeF2, I3
AX3E0 Trigonal planar BF3, CO32−, NO3, SO3
AX3E1 Trigonal pyramidal NH3, PCl3
AX3E2 T-shaped ClF3, BrF3
AX4E0 Tetrahedral CH4, PO43−, SO42−, ClO4
AX4E1 Seesaw SF4
AX4E2 Square Planar XeF4
AX5E0 Trigonal Bipyramidal PCl5
AX5E1 Square Pyramidal ClF5, BrF5
AX6E0 Octahedral SF6
AX6E1 Pentagonal pyramidal XeF6
AX7E0 Pentagonal bipyramidal IF7
† Geometry including lone pairs, shown in pale yellow
‡ Geometry excluding lone pairs

When the substituent (X) atoms are not all the same, the geometry is still approxmiately valid, but the bond angles may be slightly different than the ones where all the outside atoms are the same. For example, the double-bond carbons in alkenes like C2H4 are AX3E0, but the bond angles are not all exactly 120 °. Similarly, SOCl2 is AX3E1, but because the X substituents are not identical, the XAX angles are not all equal.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "AXE_method". A list of authors is available in Wikipedia.
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