AXE method (chemistry)

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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:

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 rwo (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.

When the outside 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, and other compounds like SOCl₂ are AX₃E₀, but the bond angles are not all exactly 120 degrees.