AX4 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 what shape and geometry the AX4 or AX4E0 generic formula belongs to. Other interesting facts about the AX4-type molecules including their examples, hybridization, polarity, etc., are also discussed.
AX4 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 AX4 or AX4E0, X = 4 so four atoms are directly bonded to the central atom A.
E=0 so there is no lone pair of electrons on the central atom in AX4-type molecules.
This shows that all 4 electron density regions or electron domains in an AX4-type molecule are constituted of bond pairs as there is no lone pair on the central atom.
AX4 molecular shape and electron geometry
AX4 molecules have a tetrahedral electron geometry and molecular geometry or shape.
The AX4 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 AX4-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., tetrahedral.
Only a bond pair-bond pair repulsive effect exists between A-X bonds. The X-atoms occupy the four corners of a tetrahedron, keeping a distance as far apart from one another as possible, as shown in the figure below.
The central atom has sp3 hybridization in AX4-type molecules.
During chemical bonding, one s atomic orbital of the central atom A mixes with three p orbitals to yield four sp3 hybrid orbitals. Each sp3 hybrid orbital has a 25% s-character and a 75% p-character.
Each of the four sp3 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 tetrahedron in AX4-type molecules.
Is an AX4-type molecule polar or non-polar?
An AX4-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 three downwards-pointing A-X bonds in the tetrahedron.
For example, methane (CH4) is a non-polar AX4-type molecule. An electronegativity difference of 0.35 units exists between the bonded carbon and hydrogen atoms in each C-H bond. Thus all four C-H bonds are individually mildly polar in the CH4 molecule.
But the dipole moments of all the C-H bonds cancel out completely in a symmetrical tetrahedral shape. The electron cloud stays uniformly distributed hence CH4 is non-polar (net µ = 0).
There are plenty of examples of tetrahedral AX4-type molecules.
These include molecules such as CH4, SiH4, and SnH4 where the central atom is bonded to four H-atoms in a tetrahedral arrangement.
Boron tetrafluoride (BF4–) is an AX4-type molecular ion. A boron (B) atom is present at the center which is bonded to four fluorine (F) atoms. The central B-atom carries a negative formal charge. However, no lone pair of electrons is present on this B-atom.
The AX4 or AX4E0 VSEPR notation represents molecules in which the central atom (A) is bonded to four other atoms (X) and it has no lone pairs of electrons (E) present on it.
How many electron density regions are there in AX4-type molecules?
The AX4-type molecules have a total of 4 electron density regions around the central atom (A). All four electron density regions or electron domains are constituted of bond pairs only.
What is the molecular geometry or shape of AX4-type molecules?
The AX4-type molecules possess a tetrahedral molecular geometry or shape.
How is the molecular shape of AX4-type molecules different from their ideal electron pair geometry?
The AX4-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 these molecules.
What is the bond angle in AX4-type molecules?
An AX4-type molecule possesses a mutual X-A-X bond angle of 109.5° in its tetrahedral shape.
Summary
AXE notation is used for determining molecular shapes as per the VSEPR concept.
The AX4 or AX4E0 generic formula represents molecules in which there are 4 atoms bonded to the central atom and there are no lone pairs of electrons present on it.
In total, 4 electron density regions are present around the central atom (A).
The ideal electronic geometry of AX4-type molecules is tetrahedral.
The molecular geometry or shape of AX4-type molecules is also tetrahedral.
AX4-type molecules are ideally non-polar.
The ideal bond angle in AX4 type molecule is 109.5º.
The central atom has sp3 hybridization in AX4-type molecules.
Examples of AX4 type molecules: CH4, CCl4, CF4, SiH4, SiCl4, SiF4, GeCl4, SnH4, SnCl4, PbCl4, BF4– etc.
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