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Chemistry
Atoms and Atomic Structure
Chemical Bonding

How do you determine if a molecule is polar or non-polar?

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November 03, 2011 5:17PM

Polar Bonds and Molecular Shape

A polar molecule is a molecule that has a net dipole moment due to its having unsymmetrical polar bonds. There are two factors that go into determining if a molecule is polar or not. To determine if a molecule (or ion) is polar or non-polar, you must determine both factors.

  • The polarity of the individual bonds in the molecule.
  • The shape or geometry of the molecule.

First, to determine if a given individual bond is polar, you need to know the electronegativity of the two atoms involved in that bond. To find the electronegativities of all the elements, look at the periodic table (follow the link below this answer under Web Links).

If the electronegativity of the two atoms has a difference of 0.3 or less, then the bond is non-polar. If the electronegativity difference is greater that 0.3 but less than 1.7, then the bond is polar. If the two values have a difference greater than 1.7, then the bond is ionic, which is just very very polar.

Once you know which bonds in the molecule are polar and which are non-polar, you must use the shape of the molecule. You need the shape because two polar bonds, if oriented correctly can cancel each other out (like two equally strong people pulling in opposite directions on a rope -- nobody moves).

The three possible outcomes:

  • If all bonds are non-polar, then the whole molecule is non-polar regardless of its shape.
  • If there is symmetry in the molecule so that the polarity of the bonds cancels out, then the molecule is non-polar. A common example of this is carbon dioxide, or CO2. The molecule is linear, and its Lewis dot structure is like this: O=C=O (this doesn't include two sets of lone pairs on each oxygen). The carbon-oxygen bond is a polar bond, but because they are exactly opposed to each other, the molecule is overall non-polar. Another example of this is CCl4, where each carbon-chlorine bond is polar, but the molecule is non-polar. Here, how they cancel out isn't as obvious, but they do. CCl4 is a tetrahedral molecule, and the 4 C-Cl polar bonds cancel each other out.
  • If there are polar bonds but there is no symmetry such that they cancel each other out, the overall molecule is polar. Water is a typical example of this. The two O-H bonds are oriented in a V-shape, and so the don't cancel out. Similarly, CH3Cl is also polar. It is the same shape as CCl4 (see above), but now it doesn't have the same symmetry because there is only one C-Cl bond and the bonds don't cancel out anymore.