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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.

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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	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.

Categories: Molecular geometry | Stereochemistry