AX3E2 Molecular geometry, VSEPR, Lone pairs, Examples, Bond angles, Polar or nonpolar

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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.

AXE VSEPR notation

  • 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.

Let us discuss through this article, some interesting facts about AX3E2 VSEPR notation including examples of AX3E2-type molecules, their shape, geometry, bond angles, hybridization, polarity, etc.

VSEPR notationAX3E2
Molecular geometry or shapeT-shaped 
Electron geometryTrigonal bipyramidal
Lone pairs (E)2
Bond pairs (X)3
Total electron density region5
Polar or nonpolarPolar 
Symmetric or asymmetric
Asymmetrical 
Hybridizationsp3d
Bond angle< 90°, < 180º
ExamplesClF3, BrF3, etc.

AX3E2 in chemistry

AX3E2 VSEPR Notation

AX3E2 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 AX3E2, X=3 so three atoms are directly bonded to the central atom A.
  • E=2 so there are 2 lone pairs of electrons on the central atom in AX3E2-type molecules.

AX3E2 VSEPR notation

AX3E2 molecular shape and electron geometry

The molecules represented by an AX3E2 generic formula possess a T-shaped.  

The ideal electronic geometry of an AX3E2-type molecule is trigonal bipyramidal.

AX3E2 electron geometry

There are a total of 5 electron density regions or electron domains in an ideal trigonal bipyramidal molecule. 2 electron domains occupy axial positions while the remaining 3 electron domains occupy equatorial positions.

If the 2 axial positions and 1 equatorial position are occupied by bonded electron pairs (X) and 2 lone pairs (E) are placed at the remaining two equatorial positions, then in addition to A-X bond pair-bond pair repulsions, the presence of lone pairs also leads to lone pair-lone pair and a lone pair-bond pair repulsive effect. The molecule consequently adopts a T-shape.

This results in the formation of AX3E2-type molecules, as shown below.

AX3E2 molecular geometry or shape

Also check:

AX3E2 Bond angle and Hybridization

The sum of all angles on a straight line is equal to 180°. However, a slight distortion in the T-shaped AX3E2-type molecules results in an X-A-X bond angle of less than 180° (approx. 175°).

AX3E2 bond angle

Also read:

The AX3E2-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.

Two of these five sp3d hybrid orbitals contain paired electrons which are situated as two lone pairs on the central A atom in an AX3E2-type molecule.

The other three 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 AX3E2-type molecule polar or non-polar?

An AX3E2-type molecule is usually polar due to its non-planar, asymmetric shape and molecular geometry.

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 AX3E2-type molecule. The distorted shape and geometry of these molecules further enhance the polarity effect.

The dipole moments of three A-X bonds do not get canceled. The electron cloud stays non-uniformly distributed in the molecule overall thus AX3E2-type molecules are polar (net µ>0).

Also check:

Examples of AX3E2-type molecules

Two well-known examples of molecules represented by AX3E2 VSEPR notation are chlorine trifluoride (ClF3) and bromine trifluoride (BrF3).

AX3E2 examples

Both the above examples consist of a halogen atom at the center which is bonded to three other halogen atoms and there are two lone pairs of electrons present on this central atom.

Both ClF3 and BrF3 are polar molecules. Each Cl-F bond in the ClF3 molecule is polar as an electronegativity difference of 0.82 units exists between the bonded Cl and F-atoms.

Cl attains a partial positive (δ+) charge while all three F-atoms obtain a partial negative (δ) charge. The electron cloud stays non-uniformly distributed in the molecule overall thus ClF3 is polar (net µ > 0).

AX3E2 polar or nonpolar

Similarly, the net dipole moment of BrF3 = 1.19 Debye.

ax3e2 polarity

Also read:

FAQ

What does AX3E2 VSEPR notation represent?

The AX3E2 VSEPR notation represents molecules in which the central atom (A) is bonded to three other atoms (X) and it has two lone pairs of electrons (E) present on it.

How many electron density regions are there in AX3E2-type molecules?

Three bond pairs and 2 lone pairs make a total of 3+2 = 5 electron density regions around the central atom in the AX3E2-type molecules.

How many bond pairs and lone pairs are there around the central atom in an AX3E2-type molecule?

There are 3 bond pairs and 2 lone pairs around the central atom in an AX3E2-type molecule.

What is the molecular geometry of AX3E2-type molecules?

The AX3E2-type molecules possess a T-shape and molecular geometry.

What is the ideal electronic geometry of AX3E2-type molecules?

The ideal electron pair geometry of AX3E2-type molecules is trigonal bipyramidal.

Summary

  • AXE notation is used for determining molecular shapes as per the VSEPR concept.
  • The AX3E2 generic formula represents molecules in which there are 3 atoms bonded to the central atom and 2 lone pairs of electrons are present on it.
  • In total, 3+2 = 5 electron density regions are present around the central atom (A).
  • The ideal electronic geometry of AX3E2-type molecules is trigonal bipyramidal.
  • The molecular geometry of AX3E2-type molecules is T-shaped.   
  • AX3E2-type molecules are ideally polar.
  • The central atom in AX3E2-type molecules has sp3d hybridization.
  • The ideal bond angle in AX3E2 type molecule is < 90°, < 180º.
  • Examples of AX3E2-type molecules are ClF3 and BrF3.  

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