AX3 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 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 AX3 or AX3E0 VSEPR notation including examples of AX3-type molecules, their shape, geometry, bond angles, hybridization, polarity, etc.
AX3 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 AX3 or AX3E0, X = 3 so three atoms are directly bonded to the central atom A.
E=0 so there is no lone pair of electrons on the central atom in AX3-type molecules.
This shows that all the 3 electron density regions or electron domains in an AX3-type molecule are constituted of bond pairs as there is no lone pair on the central atom.
AX3 molecular shape and electron geometry
AX3 molecules have a trigonal planar electron geometry and molecular geometry or shape.
The AX3 VSEPR notation represents molecules that possess an ideal, planar symmetrical shape and molecular geometry.
There is no lone pair of electrons on the central atom in AX3-type molecules or molecular ions thus there is no distortion present in their shape and geometry. These molecules consequently occupy an identical molecular and electron pair geometry i.e., trigonal planar.
Only a bond pair-bond pair repulsive effect exists between A-X bonds. The X-atoms occupy the vertices of an equilateral triangle, keeping a distance as far apart from one another as possible. Thus, the name trigonal planar or triangular planar is given to the AX3 molecular shape.
The central atom has sp2 hybridization in AX3-type molecules.
During chemical bonding, one s atomic orbital of the central atom A mix with two p orbitals to yield three sp2 hybrid orbitals. Each sp2 hybrid orbital has a 33.3% s-character and a 66.7% p-character.
Each of the three sp2 hybrid orbitals contains a single electron, it thus overlaps with the atomic orbital of a bonded atom (X) to form the A-X sigma (σ) bond on each side of the equilateral triangle in AX3-type molecules.
Is an AX3-type molecule polar or non-polar?
An AX3-type molecule is ideally non-polar.
If the bonded atoms possess an electronegativity difference greater than 0.5 units in each A-X bond, then an individual A-X bond will be polar.
However, it is due to the symmetrical shape of the molecule, that the dipole moments of an upwards-pointing A-X bond cancel with the net dipole moment of two downwards-pointing A-X bonds.
An example of an AX3-type molecule is boron trifluoride (BF3). An electronegativity difference of 1.94 units exists between the bonded B and F atoms in each B-F bond. Thus each B-F bond in the BF3 molecule is polar.
However, the dipole moments of three B-F bonds cancel out one another’s effect. The electron cloud gets uniformly distributed in the molecule overall. Consequently, BF3 is a non-polar molecule (net µ= 0).
An exception to this concept is formaldehyde (HCHO) which is a polar AX3-type molecule (net µ = 2.3 D). It consists of a C-atom at the center which is bonded to two H-atoms and one O-atom in a trigonal planar arrangement.
A C-H bond is only weakly polar but a C=O bond is strongly polar. The electronegative oxygen atom attracts the shared electron cloud of each C-H bond in addition to attracting C=O bonded electrons. The electron cloud stays non-uniformly distributed hence HCHO is polar in nature.
There are plenty of examples of trigonal planar AX3-type molecules.
These examples include neutral molecules having Boron (B) as the central atom and halogens as the outer atoms such as boron trifluoride (BF3), boron trichloride (BCl3), boron tribromide (BBr3) and boron triiodide (BI3).
Sulfur trioxide (SO3) is another AX3-type neutral molecule. Additionally, some molecular ions such as the nitrate (NO3–) ion and carbonate (CO32-) ion are also representative of AX3-type molecules.
The AX3 or AX3E0 VSEPR notation represents molecules in which the central atom (A) is bonded to three other atoms (X) and it has no lone pairs of electrons (E) on it.
How many electron density regions are there in AX3-type molecules?
The AX3-type molecules have a total of 3 electron density regions around the central atom (A). All three electron density regions or electron domains are constituted of bond pairs only.
What is the molecular geometry or shape of AX3-type molecules?
The AX3-type molecules possess a trigonal planar molecular geometry or shape.
How is the molecular shape of AX3-type molecules different from their ideal electron pair geometry?
The AX3-type molecules have an identical molecular shape to that of their electron pair geometry. As there are no lone pairs of electrons on the central atom so no distortion is present in the shape and/or geometry of AX3-type molecules.
What is the bond angle in AX3-type molecules?
An AX3-type molecule possesses a mutual X-A-X bond angle of 120° in its trigonal planar shape.
Summary
AXE notation is used for determining molecular shapes as per the VSEPR concept.
The AX3 or AX3E0 generic formula represents molecules in which there are 3 atoms bonded to the central atom and there are no lone pairs of electrons present on it.
In total, 3 electron density regions are present around the central atom (A).
The ideal electronic geometry of AX3-type molecules is trigonal planar.
The molecular geometry or shape of AX3-type molecules is also trigonal planar.
AX3-type molecules are ideally non-polar (Formaldehyde (HCHO) is an exception).
The ideal bond angle in AX3 type molecule is 120º.
The central atom has sp2 hybridization in AX3-type molecules.
Examples of AX3 type molecules: BF3, BCl3, BBr3, BI3, HCHO, SO3, NO32-, CO32- etc.
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