AXE method

Jump to: navigation, search

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:

File:VSEPR geometries.PNG
Caution: the notation in this diagram differs from the article. Here E represents the central atom (called A in the article); X represents outside atoms (same as the article); and two dots represents a lone pair (called E in the article). The last two entries on the last row (AX3E3, AX2E4) are not found in the article, and steric number 7 is not listed in the table. Entries with only one outside atom are also not listed in the table.
Type Shape Geometry Geometry Examples
AX1E* Linear (N/A) 100px 100px HF, O2
AX2E0 Linear 100px 100px BeCl2, HgCl2, CO2
AX2E1 Bent 100px 100px NO2, SO2, O3
AX2E2 Bent 100px 100px H2O, OF2
AX2E3 Linear 100px 100px XeF2, I3
AX3E0 Trigonal planar 100px 100px BF3, CO32−, NO3, SO3
AX3E1 Trigonal pyramidal 100px 100px NH3, PCl3
AX3E2 T-shaped 100px 100px ClF3, BrF3
AX4E0 Tetrahedral 100px 100px CH4, PO43−, SO42−, ClO4
AX4E1 Seesaw 100px 100px SF4
AX4E2 Square Planar 100px 100px XeF4
AX5E0 Trigonal Bipyramidal 100px 100px PCl5
AX5E1 Square Pyramidal 100px 100px ClF5, BrF5
AX6E0 Octahedral 100px 100px SF6
AX6E1 Pentagonal pyramidal 100px 100px XeF6
AX7E0 Pentagonal bipyramidal 100px 100px 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.