AX2E2 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 interesting facts about AX2E2 VSEPR notation including examples of AX2E2-type molecules, their shape, geometry, bond angles, hybridization, polarity, etc.
AX2E2 VSEPR notation represents a molecule or a molecular ion that consists of a total of 4 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 AX2E2, X=2 so two atoms are directly bonded to the central atom A.
E=2 so there are a total of 2 lone pairs of electrons on the central atom in AX2E2-type molecules.
AX2E2 molecular shape and electron geometry
The molecules represented by an AX2E2 generic formula possess a bent shape or molecular geometry.
The ideal electronic geometry of an AX2E2-type molecule is tetrahedral.
The presence of two lone pairs of electrons on the central atom A leads to strong 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 the A-X bonds tilt inwards, away from the lone pairs and towards each other. Consequently, the AX2E2-type molecules occupy a bent or angular shape and molecular geometry.
The lone pair repulsive effect due to the presence of two lone pairs of electrons on the central atom decreases the X-A-X bond angle from the ideal 109.5° (as expected in a tetrahedral molecule) to about 104.5° in the bent AX2E2 shape.
During chemical bonding, one s atomic orbital of the central atom A hybridizes with its three p orbitals to yield four sp3 hybrid orbitals. Each sp3 hybrid orbital possesses a 25% s-character and a 75% p-character.
Two of these sp3 hybrid orbitals contain paired electrons which are situated as 2 lone pairs on the central A atom.
The other two sp3 hybrid orbitals contain a single electron each which is used for sigma (σ) bond formation with the two X-atoms.
Is an AX2E2-type molecule polar or non-polar?
An AX2E2-type molecule is usually polar due to its non-planar, asymmetric, bent shape and molecular geometry.
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 AX2E2-type molecule.
For example, water (H2O) is an AX2E2-type molecule that is strongly polar. The dipole moments of two O-H bonds add up in the bent shape of H2O. The electron cloud stays non-uniformly distributed overall thus H2O is polar (net µ =1.86 Debye).
Similarly, dihydrogen sulfide (H2S) is a polar AX2E2-type molecule. The dipole moments of H-S bonds add up to yield a non-zero net dipole moment (µ = 0.95 D).
In all the above examples, the central atom is bonded to two other atoms and it consists of 2 lone pairs of electrons, forming a bent molecular arrangement.
The AX2E2 VSEPR notation represents molecules in which the central atom (A) is bonded to two other atoms (X) and it has two lone pairs of electrons (E) on it.
How many electron density regions are there in AX2E2-type molecules?
The AX2E2-type molecules have a total of 4 electron density regions around the central atom A. Two bonded atoms (X) and two lone pairs (E). So, the total is 2 + 2 = 4.
Which molecular shape and electron geometry do AX2E2-type molecules possess?
The AX2E2-type molecules possess a bent molecular geometry or shape and have a tetrahedral electron geometry.
How many bond pairs and lone pairs are there on the central atom A in AX2E2-type molecules?
There are 2 bond pairs and 2 lone pairs of electrons on the central atom A in AX2E2-type molecules.
How can the electron geometry of AX2E2-type molecules be determined from steric numbers?
Steric number = Total electron density regions or electron domains around the central atom in a molecule.
The steric number of the central atom A in AX2E2 –type molecules is 2+2 = 4. Thus, it has a tetrahedral electron geometry.
Summary
AXE notation is used for determining molecular shapes as per the VSEPR concept.
The AX2E2 generic formula represents molecules in which there are 2 atoms bonded to the central atom and 2 lone pairs of electrons are present on it.
In total, 2+2 = 4 electron density regions are present around the central atom (A).
The ideal electronic geometry of AX2E2-type molecules is tetrahedral.
The molecular geometry or shape of AX2E2-type molecules is bent.
AX2E2-type molecules are generally polar.
The ideal bond angle in an AX2E2-type molecule is less than 109.5º.
The central atom has sp3 hybridization in AX2E2-type molecules.
Examples of AX2E2 type molecules: H2O, H2S, OF2, SCl2, OCl2, etc.
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