AX2E or AX2E1 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 learn what shape and geometry the AX2E or AX2E1 generic formula belongs to. Other interesting facts about the AX2E-type molecules including their examples, hybridization, polarity, etc., are also discussed. So, continue reading!
AX2E VSEPR notation represents a molecule or a molecular ion that consists of a total of 3 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 AX2E, X=2 so two atoms are directly bonded to the central atom A.
E=1 so there is only one lone pair of electrons on the central atom in AX2E or AX2E1-type molecules.
AX2E molecular shape and electron geometry
The molecules represented by an AX2E generic formula possess a bent or V-shape and molecular geometry.
The ideal electronic geometry of an AX2E-type molecule is trigonal planar.
The presence of a lone pair of electrons on the central atom A leads to 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 away. A-X bonds tilt inwards and the molecule occupies a bent shape, as shown in the figure below.
As the A-X bonds tilt inwards, the X-A-X bond angle decreases from the ideal 120° (as expected in a trigonal planar molecule) to about 119° in AX2E-type molecules.
The central atom (A) is sp2 hybridized in a bent AX2E-type molecule.
The sp2 hybridization corresponds to a steric number of 3 i.e., a total of 3 regions of electron density or electron domains surrounding the central atom in the molecule.
During chemical bonding, one s atomic orbital of atom A mixed with two p orbitals to yield three sp2 hybrid orbitals.
One of these three hybrid orbitals contains paired electrons which are situated as a lone pair on A.
While the remaining two sp2 hybrid orbitals contain a single electron each. These sp2 hybrid orbitals overlap with the atomic orbitals of bonded atoms (X) to form the two required A-X sigma (σ) bonds in AX2E.
Is an AX2E-type molecule polar or non-polar?
An AX2E-type molecule is predominantly polar (net dipole moment, µ > 0). Bent or V-shape is a 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 two A-X bonds does not get canceled in the bent shape. Rather, the polarity effect adds up to yield a polar AX2E-type molecule.
For example, SO2 an AX2E-type molecule is strongly polar. The dipole moments of S=O bonds add up in the bent shape of SO2. The electron cloud stays non-uniformly distributed overall thus SO2 is polar (net µ =1.62 Debye).
Sulfur dioxide (SO2), stannous chloride (SnCl2), and lead chloride (PbCl2) are some well-known examples of AX2E-type molecules. In each of these examples, the central atom is bonded to two other atoms while a lone pair of electrons is present at the top of the central atom.
The AX2E or AX2E1 VSEPR notation represents molecules in which the central atom (A) is bonded to two other atoms (X) and it has a lone pair of electrons (E) on it.
How many total electron density regions are there in the AX2E-type molecules?
The AX2E1-type molecules have a total of 3 electron density regions around the central atom A. Two bonded atoms (X) and one lone pair (E). So, the total is 2 + 1 = 3.
What molecular shape do the AX2E-type molecules possess?
The AX2E-type molecules possess a bent molecular shape. It is also known as angular or V-shape.
What is the ideal electron pair geometry of AX2E-type molecules?
The ideal electron pair geometry of AX2E-type molecules is trigonal planar. This corresponds to a steric number of 3 of the central A-atom.
Three electron density regions arranged along the vertices of an equilateral triangle lead to the formation of a trigonal planar molecule.
Why do AX2E-type molecules occupy a different shape from their ideal electronic geometry?
A lone pair of electrons present on the central A-atom leads to lone pair-bond pair electronic repulsions.
This repulsive effect distorts the geometry of the AX2E-type molecules and makes them occupy a bent shape, different from their ideal trigonal planar electron geometry.
Summary
AXE notation is used for determining molecular shapes as per the VSEPR concept.
The AX2E or AX2E1 generic formula represents molecules in which there are 2 atoms bonded to the central atom and 1 lone pair of electrons is also present on it.
In total, 2+1 = 3 electron density regions are present around the central atom (A).
The ideal electronic geometry of AX2E-type molecules is trigonal planar.
The molecular geometry or shape of AX2E-type molecules is bent.
AX2E-type molecules are generally polar.
The ideal bond angle in AX2E type molecule is less than 120º.
AX2E-type molecules have sp2 hybridization.
Examples of AX2E type molecules: SO2, SnCl2, PbCl2 etc.
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