AX4E2 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 AX4E2 generic formula belongs to. Other interesting facts about the AX4E2-type molecules including their examples, hybridization, polarity, etc., are also discussed.
AX4E2 VSEPR notation represents a molecule or a molecular ion that consists of a total of 6 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 AX4E2, X=4 so four atoms are directly bonded to the central atom A.
E=2 so there are a total of two lone pairs of electrons on the central atom in AX4E2-type molecules.
AX4E2 molecular shape and electron geometry
The molecules represented by an AX4E2 generic formula possess a square planar shape or molecular geometry.
The ideal electronic geometry of an AX4E2-type molecule is octahedral.
The presence of two 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.
To minimize the strong repulsive effect, two lone pairs thus occupy opposite positions in an AX4E2-type molecule resulting in a square planar shape.
The bonded atoms (X) lie at the four vertices of the square while the non-bonded electrons are placed above and below this square, as shown in the figure below.
The central atom (A) has sp3d2 hybridization in AX4E2-type molecules.
During chemical bonding, one s atomic orbital of the central atom hybridizes with three p and two d orbitals to yield six sp3d2 hybrid orbitals.
Two sp3d2 hybrid orbitals contain paired electrons which are situated as two lone pairs in an AX4E2 square planar arrangement.
The other four sp3d2 hybrid orbitals contain a single electron each which they use for sigma (σ) bond formation by overlapping with the atomic orbitals of the bonded atoms.
Is an AX4E2-type molecule polar or non-polar?
An AX4E2-type molecule is ideally non-polar accredited to its symmetrical square planar shape.
For example, xenon tetrafluoride (XeF4) is an AX4E2-type molecule. It consists of four Xe-F bonds and two lone pairs of electrons are present on the central Xe atom. An electronegativity difference of 1.38 units exists between the bonded Xe and F atoms.
Thus, each Xe-F bond is individually polar in the XeF4 molecule and possesses a specific dipole moment value (symbol µ).
However, it is due to the symmetrical square planar shape of XeF4 that the dipole moments of two oppositely pointing Xe-F bonds cancels each other’s effect completely. Consequently, XeF4 is a non-polar molecule overall (net µ =0).
Other than xenon tetrafluoride (XeF4) which is a neutral AX4E2-type molecule, another prominent example of an AX4E2-type molecular ion is iodine tetrachloride (ICl4–).
The AX4E2 VSEPR notation represents molecules in which the central atom (A) is bonded to four other atoms (X) and it has two lone pairs of electrons (E) on it.
How many electron density regions are there in AX4E2-type molecules?
The AX4E2-type molecules have a total of 6 electron density regions around the central atom A. Four bonded atoms (X) and two lone pairs (E). So, the total is 4 + 2 = 6.
What is the molecular geometry of AX4E2-type molecules?
The AX4E2-type molecules possess a square planar shape or molecular geometry.
What is the ideal electronic geometry of AX4E2-type molecules?
The ideal electronic geometry of AX4E2-type molecules is octahedral, corresponding to a steric number of 6.
The six electron density regions or electron domains arranged around the six corners of an eight vertices arrangement form an octahedron.
Why is the molecular shape of AX4E2-type molecules different from their ideal electronic geometry?
The presence of two lone pairs of electrons on the central atom (A) leads to a lone pair-lone pair and lone pair-bond pair repulsive effect which is stronger than the bond pair-bond pair electronic repulsions.
Thus, AX4E2-type molecules attain a different molecular shape than their ideal electronic geometry i.e., square planar vs octahedral.
Summary
AXE notation is used for determining molecular shapes as per the VSEPR concept.
The AX4E2 generic formula represents molecules in which there are 4 atoms bonded to the central atom and 2 lone pairs are present on it.
In total, 4+2 = 6 electron density regions are present around the central atom (A).
The ideal electronic geometry of AX4E2-type molecules is octahedral.
The molecular geometry or shape of AX4E2-type molecules is square planar.
AX4E2-type molecules are usually non-polar.
AX4E2-type molecules have sp3d2 hybridization.
Examples of AX4E2 type molecules: XeF4, ICl4–, etc.
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The presence of two 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.
