How to Determine a Molecule's Polarity

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Molecular polarity occurs when atoms with different electronegativity rates combine in a fashion that results in an unsymmetrical distribution of electrical charge. Since all atoms have a certain amount of electronegativity, all molecules are said to be somewhat dipole. However, when a molecule possesses a symmetrical structure, the charges cancel each other out, thus resulting in a non-polar molecule. The same thing happens when all the atoms in a molecule contain the same electronegativity.

    Determine each atom's electronegativity by using a periodic table of elements. If all the atoms have the same electronegativity, then the molecule is by default non-polar. Given the molecule CH4, Carbon (C) has an electronegativity of 2.5 and Hydrogen (H) has one of 2.1. Given the molecule NH3, Nitrogen (N) has an electronegativity of 3.0. However, given the molecule NCl3, Nitrogen and Chlorine both have the same electronegativity of 3.0, so the molecule is non-polar.

    Draw the molecule using the Lewis dot diagram method. Count the number of valence electrons each atom contains. Arrange the atoms so that the one with the greatest electronegativity is in the center. Connect the atoms with single electron bonds and remove these electrons from the valence count. Position pairs of electrons around the outer atoms until you achieve an octet, and then remove these electrons from the count. Place any remaining electrons around the atom in the center.

    Determine the molecule's polarity by inspecting its shape for symmetry. Given the example, the molecule CH4 has a tetrahedral shape that is symmetric. Thus, it is non-polar. The molecule NCl3 has a pyramidal shape, on the other hand, so it is polar. In general, molecules with linear, trigonal and tetrahedral shapes are non-polar, while atoms with pyramidal and V-shaped shapes are polar.


About the Author

Vivek Saxena has been a full-time freelance writer since 2004, contributing to several online publications. Prior to becoming a writer, Saxena studied computer technology at Purdue University.

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