Determining the polar or non-polar character of a molecule or compound is important in deciding what kind of solvent to use to dissolve it. Polar compounds only dissolve in polar solvents and non-polar in non-polar solvents. While some molecules like ethyl alcohol dissolve in both types of solvents, the former statement is a good rule of thumb to follow. Determining the polar character of a compound uses the concept of dipole moments of bonds and spatial geometry of the compound.
Draw a Lewis dot structure for the compound of interest. Identify each region of negative charge. Zones of negative charge reside in the bonds and on the lone pairs of electrons present in the compound.
Assign a dipole moment to each bond of the molecule. The magnitude of the dipole depends on the difference in the electronegativities of the two atoms. Lone pairs of electrons have a negative charge in the direction exactly opposite of the nucleus of the atom.
Convert the Lewis dot structure to a spatial mole of the molecule with bonds situated according to Valence Shell Electron Pair Repulsion (VSEPR) theory. Atoms with four electron pairs form a tetrahedral orientation, atoms with a double bond are trigonal planar bonds and triple bond molecules are linear.
Determine the overall dipole of the compound. Add each dipole moment of each bond to form an overall dipole moment for the molecule. Symmetry of the compound indicates whether there is a dipole moment for the compound. If the molecule is symmetrical, then there is no dipole because the dipole moments won't cancel out.
Classify the compound as polar if an overall dipole moment exists for the compound and is non-polar if no overall dipole moment exists.
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