How do you determine if a molecule non polar?

1 Answer
Nov 15, 2016

A molecule is non-polar iff the #Deltachi# (the diff. in electronegativity between each bonded atom) is below #0.5# and/or iff the molecular shape of the atom is symmetrical.


There are 2 different types we're talking about.

Intramolecular forces: The net electronegativity, which is calculated by subtracting the element with the lower electronegativity from the element with the higher electronegativity. If that difference is below #0.5# then you have a non-polar bond. It's important that bond is mentioned because even though the bond is non-polar, the molecule itself may be polar.

VSEPR shape/atom symmetry: After determining your covalent bond (because only molecules with a #Deltachi# of below 0.5 will be non-polar molecular compounds), you must draw out the VSEPR diagram. Not going in depth with that, but basically, if your central atom is surrounded by atoms of the same element, like methane (#"CH"_4#), then the dipoles cancel out and your molecule is non-polar, in general.

In summary, if the difference in electronegativity is below #0.5#, then your molecule has a non-polar bond. Afterwards, draw your VSEPR diagram and if it's symmetrical (dipoles cancel out), then the molecule is non-polar in general.

Some examples:

  • Methane (#"CH"_4#) has a net #chi# of 0.4 - it has non-polar covalent bonds. It's also symmetrical thus it is a non-polar molecule (in general).
  • Boron trichloride (#"BCl"_3#) has a net #chi# of 1 - it has polar covalent bonds. However, it is symmetrical, thus it is non-polar in general but has polar covalent bonds.