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 = sp^{2}, 4 = sp^{3}, 5 = sp^{3}d, 6 = sp^{3}d^{2}, 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 |
---|---|---|---|---|
AX_{1}E_{*} | Linear (N/A) | 100px | 100px | HF, O_{2} |
AX_{2}E_{0} | Linear | 100px | 100px | BeCl_{2}, HgCl_{2}, CO_{2} |
AX_{2}E_{1} | Bent | 100px | 100px | NO_{2}^{−}, SO_{2}, O_{3} |
AX_{2}E_{2} | Bent | 100px | 100px | H_{2}O, OF_{2} |
AX_{2}E_{3} | Linear | 100px | 100px | XeF_{2}, I_{3}^{−} |
AX_{3}E_{0} | Trigonal planar | 100px | 100px | BF_{3}, CO_{3}^{2−}, NO_{3}^{−}, SO_{3} |
AX_{3}E_{1} | Trigonal pyramidal | 100px | 100px | NH_{3}, PCl_{3} |
AX_{3}E_{2} | T-shaped | 100px | 100px | ClF_{3}, BrF_{3} |
AX_{4}E_{0} | Tetrahedral | 100px | 100px | CH_{4}, PO_{4}^{3−}, SO_{4}^{2−}, ClO_{4}^{−} |
AX_{4}E_{1} | Seesaw | 100px | 100px | SF_{4} |
AX_{4}E_{2} | Square Planar | 100px | 100px | XeF_{4} |
AX_{5}E_{0} | Trigonal Bipyramidal | 100px | 100px | PCl_{5} |
AX_{5}E_{1} | Square Pyramidal | 100px | 100px | ClF_{5}, BrF_{5} |
AX_{6}E_{0} | Octahedral | 100px | 100px | SF_{6} |
AX_{6}E_{1} | Pentagonal pyramidal | 100px | 100px | XeF_{6} |
AX_{7}E_{0} | Pentagonal bipyramidal | 100px | 100px | IF_{7} |
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 C_{2}H_{4} are AX_{3}E_{0}, but the bond angles are not all exactly 120 °. Similarly, SOCl_{2} is AX_{3}E_{1}, but because the X substituents are not identical, the XAX angles are not all equal.