Polarity describes the tendency of a substance to have a molecular dipole, or a positively and a negatively charged end. Polar molecules are made of elements with different electronegativities, or electron attractions, meaning that one element possesses the shared electrons more often than the other. This gives the more electronegative element a partially negative charge and the more electropositive element a partially positive charge. If these elements are arranged symmetrically, so that these charges cancel one another, the molecule is non-polar. If they are arranged asymmetrically, however, they form a polar molecule.
Examining a diagram of the spatial arrangement of atoms that form a molecule can tell you if it is polar or non-polar. A molecule has polar bonds if there is a significant difference in electronegativity between the two elements. If the electronegativities of both elements are very similar or the same, the bonds are non-polar. If this is the case, the entire molecule is also non-polar. If it has polar bonds, you must examine the molecule further to determine whether it is polar or not.
Begin by drawing a Lewis diagram of the molecule. In this kind of diagram, the molecule's constituent elements are represented by their chemical symbols surrounded by dots representing their outer electrons. When properly drawn, Lewis diagrams show the number of bonds and lone pairs, or unbonded pairs of electrons, present in the molecule.
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Examine the shape of the molecule, including the number of bonds and lone pairs around the central atom. For instance, two bonds and two lone pairs create a bent molecule. Four bonds and no lone pairs create a tetrahedral shape. You may need to refer to a molecular geometry chart if you are uncertain about the shape of your molecule.
Draw a shape diagram showing how the elements are spatially arranged. If the bonds are symmetrical, their polarities cancel each other and the molecule is non-polar. If the bonds are asymmetrical, so that the more electronegative element is on one end and the electropositive element is on the other, the molecule is polar.
Mixing a liquid of unknown polarity with water can tell you if the molecules in the liquid are polar or non-polar. Simply mix the liquid with an equal part of water and allow the mixture to sit undisturbed. Examine the mixture after the liquids have sat together for a time. If they have not separated, but have formed a solution, the unknown liquid is polar. If there is a clear boundary between the two liquids, it is non-polar. For instance, oil, a non-polar molecule, always separates out of a water-based solution. However, vinegar, a polar substance, does not.