AX2E3 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 and the molecular geometry or shape of a molecule as per the Valence Shell Electron Pair Repulsion (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 AX2E3 generic formula belongs to. Other interesting facts about the AX2E3-type molecules including their examples, hybridization, polarity, etc., are also discussed.
AX2E3 VSEPR notation represents a molecule or a molecular ion that consists of a total of 5 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.
In AX2E3, X=2 so two atoms are directly bonded to the central atom A.
E=3 so there are a total of 3 lone pairs of electrons on the central atom in AX2E3-type molecules.
AX2E3 molecular shape and electron geometry
The molecules represented by an AX2E3 generic formula possess a linear shape or molecular geometry.
The ideal electronic geometry of an AX2E3-type molecule is trigonal bipyramidal.
The presence of three lone pairs of electrons on the central atom A leads to lone pair-lone pair and lone pair-bond pair electronic repulsions in the molecule, in addition to the A-X bond pair-bond pair repulsive effect.
The strong lone pair repulsive effect pushes the bonded atoms such that they occupy axial positions, furthest away from each other. The three lone pairs are then situated at equatorial positions, giving the molecule a linear shape overall.
As the sum of all angles on a straight line is always equal to 180°. Therefore, the bonded atoms form a mutual bond angle of 180° in the linear AX2E3 shape.
In AX2E3-type molecules, the central atom A is sp3d hybridized which corresponds to a steric number of 5.
An sp3d hybrid orbital is formed by the combination of one s atomic orbital of the central atom (A) with three p-orbitals and one d-orbital. This results in the formation of five sp3d hybrid orbitals.
These hybrid orbitals are not equivalent, rather they consist of one group of two equivalent oppositely directed orbitals that contain a single electron each. These oppositely directed orbitals overlap with the atomic orbitals of bonded atoms (X) to form the required sigma (σ) bonds.
The second group of three equivalent orbitals contains 2 electrons each and is situated as three lone pairs in the AX2E3-type molecule. This leads to the formation of a triangular base and two pyramids, one at the top and the other at the bottom of the molecule.
Is an AX2E3-type molecule polar or non-polar?
The polarity of AX2E3-type molecules is a complicated topic. A symmetrical linear shape implies that AX2E3-type molecules should ideally be non-polar.
But there is evidence of polar character in AX2E3-type molecules as well such as in the triiodide ion (I3–).
Let’s understand this concept with the help of some examples.
Xenon difluoride (XeF2) is an example of an AX2E3-type molecule. An electronegativity difference of 1.39 units exists between the bonded Xe and F atoms in a Xe-F bond.
Therefore, both Xe-F bonds are individually polar and possess a specific dipole moment (symbol µ). However, these oppositely pointing dipole moments get canceled equally in the linear shape of the XeF2 molecule. Thus, XeF2 is overall non-polar (net µ =0).
Contrarily, AX2E3-type molecular ions such as I3– ion are partially polar as it readily dissolve in polar solvents such as water. Negative one formal charge on these ions leads to an overall non-uniform charge distribution in the ions hence accounting for their polar character.
Each of these examples consists of a halogen atom at the center which is bonded to two other different halogen atoms at the sides while three lone pairs of electrons are present on the central atom which leads to the formation of a linear molecule.
The AX2E3 VSEPR notation represents molecules in which the central atom (A) is bonded to two other atoms (X) and it has three lone pairs of electrons (E) on it.
How many electron density regions are there in AX2E3-type molecules?
The AX2E3-type molecules have a total of 5 electron density regions around the central atom A. Two bonded atoms (X) and three lone pairs (E). So, the total is 2 + 3 = 5.
Which molecular shape and electron geometry do AX2E3-type molecules possess?
The AX2E3-type molecules possess a linear molecular geometry or shape and have a trigonal bipyramidal electronic geometry.
How can the hybridization of AX2E3-type molecules be determined from steric numbers?
Steric number of the central atom in a molecule = Bond pairs + Lone pairs
The steric number of AX2E3-type molecules = 2+3 = 5 thus they have sp3d hybridization.
Summary
AXE notation is used for determining molecular shapes as per the VSEPR concept.
The AX2E3 generic formula represents molecules in which there are 2 atoms bonded to the central atom and 3 lone pairs are present on it.
In total, 2+3 = 5 electron density regions are present around the central atom (A).
The ideal electronic geometry of AX2E3-type molecules is trigonal bipyramidal.
The molecular geometry or shape of AX2E3-type molecules is linear.
AX2E3-type molecules are usually non-polar.
The ideal bond angle for AX2E3 type molecule is 180º.
AX2E3-type molecules have sp3d hybridization.
Examples of AX2E3 type molecules: XeF2, ICl2–, IF2–, I3– , etc.
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