AX4E or AX4E1 Molecular geometry, 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 AX4E or AX4E1 generic formula belongs to. Other interesting facts about the AX4E-type molecules including their examples, hybridization, polarity, etc., are also discussed.
AX4E 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 AX4E, X=4 so four atoms are directly bonded to the central atom A.
E=1 so there is only 1 lone pair of electrons on the central atom in AX4E-type molecules.
AX4E molecular shape and electron geometry
The molecules represented by an AX4E generic formula possess a seesaw or distorted tetrahedral shape or molecular geometry.
The ideal electronic geometry of an AX4E-type molecule is trigonal bipyramidal.
The presence of one 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 distorts the shape and geometry of AX4E-type molecules. The bonding regions are pushed by the non-bonding region such that the lone pair of electrons occupy an equatorial position at the top of the molecule while the remaining four positions are occupied by the four bonding pairs.
The AX4E-type molecule thus occupies a seesaw molecular shape. The name seesaw is given with reference to the playground swing see-saw.
More than one different X-A-X bond angle is formed in a seesaw-shaped AX4E-type molecule.
For instance, sulfur tetrafluoride (SF4) is a typical example of an AX4E-type molecule. The sulfur (S) atom is present at the center, it has a lone pair of electrons and four fluorine atoms are bonded to it.
It has three different bond angles i.e., 101.6°, 173.1°, and 187° respectively, as shown in the figure below.
The AX4E-type molecules possess sp3d hybridization.
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.
One of these five sp3d hybrid orbitals contains paired electrons which are situated as a lone pair on the central A atom in an AX4E-type molecule.
The other sp3d 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 (X).
Is an AX4E-type molecule polar or non-polar?
The non-planar, asymmetric shape of AX4E-type molecules suggests that they are ideally polar in nature.
If the bonded atoms possess an electronegativity difference greater than 0.5 units in each A-X bond, then each individual A-X bond will be polar in an AX4E-type molecule. The distorted shape and geometry of these molecules further enhance the polarity effect.
The dipole moments of four A-X bonds do not get canceled. The electron cloud stays non-uniformly distributed in the molecule overall thus AX4E-type molecules are polar (net µ>0).
For example, tellurium chloride (TeCl4) is a polar AX4E-type molecule. An electronegativity difference of 1.06 units exists between the bonded Te and Cl atoms in a Te-Cl bond. The net dipole moment µ for the TeCl4 molecule is greater than zero.
Other than SF4 and TeCl4, another important example of an AX4E-type neutral molecule is SCl2F2.
While IO2F2– and IF4+ are AX4E-type molecular ions with a negative one and a positive formal charge respectively and a seesaw shape. Four F-atoms are bonded to a central I-atom and it has 1 lone pair of electrons present on it.
The AX4E or AX4E1 VSEPR notation represents molecules in which the central atom (A) is bonded to four other atoms (X) and it has one lone pair of electrons (E) present on it.
How many electron density regions are there in AX4E-type molecules?
Four bond pairs and 1 lone pair make a total of 4+1 = 5 electron density regions around the central atom in the AX4E-type molecules.
Which molecular shape and electron geometry do AX4E-type molecules possess?
The AX4E-type molecules possess a seesaw shape and have a trigonal bipyramidal electron pair geometry.
Why is the molecular shape of AX4E-type molecules different from its electronic geometry?
The presence of a lone pair of electrons on the central atom in AX4E-type molecules leads to strong lone pair-bond pair electronic repulsions which distort the shape and geometry of these molecules.
Summary
AXE notation is used for determining molecular shapes as per the VSEPR concept.
The AX4E generic formula represents molecules in which there are 4 atoms bonded to the central atom and 1 lone pair is present on it.
In total, 4+1 = 5 electron density regions are present around the central atom (A).
The ideal electronic geometry of AX4E-type molecules is trigonal bipyramidal.
The molecular geometry or shape of AX4E-type molecules is seesaw.
AX4E-type molecules are ideally polar.
The central atom in AX4E-type molecules has sp3d hybridization.
Examples of AX4E type molecules: SF4, TeCl4, SCl2F2, IF4+, IO2F2–, etc.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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