AX3E or AX3E1 Molecular shape, VSEPR, Lone pairs, Examples, Bond angles, Polar or nonpolar
The AXE notation is a general molecular formula that is used to determine the ideal electronic geometry as well as the definite shape of a molecule, as per the VSEPR concept.
A in the AXE formula represents the central atom of the molecule.
X stands for the number of atoms bonded to the central atom of the molecule.
E denotes the unbonded electrons or the lone pairs of electrons present on the central atom i.e., A.
In this article, you will find what shape and geometry the AX3E generic formula belongs to. Other interesting facts about the AX3E1-type molecules including their examples, hybridization, polarity, etc., are also discussed.
VSEPR notation
AX3E or AX3E1
Molecular geometry or shape
Trigonal pyramidal
Electron geometry
Tetrahedral
Lone pairs (E)
1
Bond pairs (X)
3
Total electron density region
4
Polar or nonpolar
Usually Polar
Symmetric or asymmetric
Asymmetric molecule
Hybridization
Sp3
Bond angle
< 109.5º
Examples
NH3 , PCl3, PH3, AsCl3, SbH3, BiH3, H3O+, ClO3– etc.
AX3E VSEPR notation represents a molecule or a molecular ion that consists of a total of 4 electron density regions around the central atom A.
Electron density regions = Number of bonded atoms (X) + lone pairs (E)
The sum of X and E is also sometimes known as the steric number of the central atom in a molecule.
X=3 so in AX3E three atoms are directly bonded to the central atom A.
E=1 so there is one lone pair of electrons on the central atom.
AX3E molecular shape and electron geometry
The molecules represented by an AX3E generic formula possess a trigonal pyramidal shape or molecular geometry.
The ideal electronic geometry of an AX3E-type molecule is tetrahedral.
It is due to the presence of a lone pair of electrons on the central atom in these types of molecules that lone pair-bond pair electronic repulsions exist in the molecule in addition to bond pair-bond pair repulsions.
As lone pair-lone pair repulsions are stronger than bond pair-bond pair repulsions. So the A-X bonds are pushed away from the lone pair of electrons. The bonded atoms occupy three corners of a triangular base while a pyramid is formed at the top.
Hence, the name trigonal pyramidal is given, as shown in the figure below.
In AX3E-type molecules, the central atom A is sp3 hybridized which corresponds to a steric number of 4.
During chemical bonding, one s atomic orbital hybridizes with three p orbitals to produce four sp3 hybrid orbitals. Each sp3 hybrid orbital possesses a 25% s-character and a 75% p-character.
In the AX3E-type molecule, three sp3 hybrid orbitals overlap with the concerned orbital of the bonded atom (X) to form the required sigma (σ) bonds. The fourth sp3 hybrid orbital contains paired electrons which are situated as a lone pair (E) on the central atom.
Is an AX3E-type molecule polar or non-polar?
An AX3E-type molecule is generally polar (net dipole moment, µ > 0). Trigonal pyramidal is non-planar, asymmetric shape.
In an A-X bond where the bonded atoms possess an electronegativity difference greater than 0.5 units, one atom occupies a partial positive (δ+) charge while the other atom attains a partial negative (δ–) charge.
The dipole moment effect of all three A-X bonds adds up to make AX3E a polar molecule overall.
To a nitrogen (N) atom at the center, three atoms of hydrogen (H) are covalently bonded and there is a lone pair of electrons on the nitrogen atom.
The H-atoms lie at the three corners of a triangular base while a lone pair is situated at the apex, forming a pyramid. NH3 is strongly polar (net µ=1.47 Debye)and has sp3 hybridization.
The AX3E or AX3E1 VSEPR notation represents molecules in which the central atom (A) is bonded to three other atoms (X) and it has a lone pair of electrons (E) on it.
How many electron density regions are there in AX3E-type molecules?
The AX3E-type molecules have a total of 4 electron density regions around the central atom A. Three bonded atoms (X) and one lone pair (E). So, the total is 3 + 1 = 4.
Which molecular shape and electron geometry do AX3E-type molecules possess?
The AX3E-type molecules possess a trigonal pyramidal shape and have a tetrahedral electron pair geometry.
Why is the molecular geometry of AX3E molecules different from their ideal electron pair geometry?
The presence of one lone pair on the central atom A in AX3E molecules leads to lone pair-bond pair repulsions.
This repulsive effect distorts the shape and geometry of these types of molecules. Consequently, they occupy a different shape from their ideal electron pair geometry.
Summary
AXE notation is used for determining molecular shapes as per the VSEPR concept.
The AX3E or AX3E1 generic formula represents molecules in which there are 3 atoms bonded to the central atom and 1 lone pair is present on it.
In total, 3+1 = 4 electron density regions are present around the central atom (A).
The ideal electronic geometry of AX3E-type molecules is tetrahedral.
The molecular geometry or shape of AX3E-type molecules is trigonal pyramidal.
AX3E-type molecules are generally polar.
The ideal bond angle in AX3E type molecule is less than 109.5º.
The central atom A is sp3 hybridized as an AX3E-type molecule so it has sp3 hybridization.
Examples of AX3E1 type molecule: NH3, PH3, PCl3, H3O+, etc.
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