About AXE Notation
AXE notation is a short hand method of representing the composition of a molecule in terms of its atoms. All molecules used in this Concept Builder have a
central atom to which all other atoms are bonded. That central atom may also have some
lone pairs (unbonded pairs) of electrons surrounding it. AXE notation identifies the number of atoms bonded to the central atom and the number of lone pairs surrounding the central atom.
The letter
A of AXE notation refers to the central atom. The letter
X refers to atoms bonded to the central atom. X has a subscripted number following it that indicates the number of bonded atoms in the molecule. The letter
E of AXE represents the lone pairs surrounding the central atom and the subscripted number following the X refers to the number of lone pairs.
AXE notation is important because it determines the molecular geometry of the molecule ... which in turn is used to determine if the molecular is polar or not.
Polar Bonds and Dipole Moments
Polar covalent bonds are bonds between two atoms in which the shared electrons are not shared equally. One of the atoms - the one which is more electronegative - exerts a greater pull upon the shared electrons than the other atom. Polar bonds will be observed when the difference in electronegativity between bonded atoms is 0.5 or greater (and less than 2.0).
The unequal sharing leads to a partial charge upon the two bonded atoms. The more electronegative atom acquires a partial negative charge; the more electropositive atom acquires a partial positive charge. A
dipole is created as a result of this unequal sharing. A dipole moment vector describe this dipole; the dipole moment is directed from the more electropositive element towards the more electronegative element.
Overall Polarity
You must become comfortable distinguishing between a
polar bond and a
polar molecule. These are two separate ideas. A bond is polar if the shared electrons are not shared equally. The distribution of electric charge is distorted in the bond with more negative charge being closer to the more electronegative element. But elsewhere in the molecule there could be a second polar bond (... or a third ... or a fourth ... or a ...) that offsets the influence of the first polar bond. If two or more polar bonds can offset the influence of one another, then the molecule itself is a non-polar molecule. Of course, if a molecule does not have any polar bonds at all, then the molecule is non-polar. But even if there are polar bonds present in a molecule, they can have the overall effect of cancelling each other.
The Importance of Molecular Geometry
We often refer to the dipole moment vectors as cancelling each other. The dipole moment vectors are directed along the bond that stretches from one atom's center to the other atom's center. Certain molecular geometries allow for such cancellation. The diagram below depicts two cases in which dipole moment vectors cancel each other.
In the case of the
linear geometry, the dipoles A and B are directed opposite one another. As such it is a bit more intuitive to assert that they would cancel each other's effects. In the
trigonal planar geometry, dipoles A ad B combine to cancel the effect of dipole C. Vector R (in blue) represents the combined effect of A + B and it is directed opposite of dipole C.
For many students of a first-time Chemistry course, the mathematical background (vector mathematics) is not sufficient to provide a full understanding of the topic. The table below is provided to assist such students. It shows the AXE notation, the corresponding molecular geometry as predicted by VSEPR (
Valence Shell Electron Pair Repulsion theory), and a column indicating whether or not the dipole moment vectors would cancel. When they do cancel, the molecule will be non-polar even if there are polar bonds present in the molecule.
An Effective Strategy
The most effective strategy involves first determining if the molecule has polar bonds. In this question, you are told that all bonds are polar. (If you weren't told this, then you would need to check an electronegativity table. If the EN values for the two bonded atoms is greater than 0.4 and less than 2.0, then the bond is polar.) The second step is to determine if the molecular geometry allows for the cancellation of dipoles. If there are no polar bonds, then the molecule is non-polar. And if there are polar bonds, the molecule can still be non-polar if the geometry allows for cancellation of dipoles (see Table above).