AX3E

AX3E is a notation used in VSEPR theory to describe a molecular geometry where a central atom A is bonded to three X atoms and has one lone pair of electrons (E). The four electron domains around A adopt a tetrahedral arrangement, giving an electron-domain geometry of tetrahedral and a molecular geometry of trigonal pyramidal. The central atom typically exhibits sp3 hybridization.

The presence of the lone pair slightly compresses the bond angles from the ideal 109.5 degrees, often to around 107 degrees in ammonia; the exact angle varies with the substituents and their electronegativities. This lone pair also causes the three bonds to orient in a pyramidal shape, with the lone pair occupying the fourth vertex of the tetrahedron.

Common examples of AX3E species include ammonia (NH3), phosphorous trichloride (PCl3), arsenic trichloride (AsCl3), and antimony trichloride (SbCl3). These molecules are typically polar because the lone pair and the three substituents create an asymmetric charge distribution. The dipole moment and overall polarity depend on the identities of the X groups.

In summary, AX3E characterizes a tetrahedral electron framework with three bonds and one lone pair, producing a trigonal pyramidal molecular geometry, typically sp3 hybridized, with bond angles slightly less than 109.5° and generally polar throughout practical examples.