Topics Covered: Polarity, bond polarity, molecular polarity, electronegativity
In chemistry and biology, “polarity” is a concept that we encounter often. Even in our previous posts when we talked about water and intermolecular forces, polarity came up during our discussion. But what does it exactly mean when a molecule is polar? How can we determine the molecule’s polarity? These are the questions that we will address today. (And sorry, polarity is not related to polar bears…)
In chemistry, polarity refers to the separation of electric charges. There are two types of polarity: bond polarity and molecular polarity. Bond polarity is simple. If two elements have difference in electronegativity and are therefore sharing the electrons unequally, we say there is a bond polarity. Typically, if the difference in electronegativity (and we’ll simply call this ΔEN) is between 0.3 and 1.7, there is a polar bond. If ΔEN is below 0.3, it’s nonpolar.
What if ΔEN is greater than 1.7? This indicates that the difference in electronegativity is so great that electrons will not be shared, but rather be completely transferred from one atom to another— in other words, the atoms will form an ionic bond!!
Question, how do we calculate the exact ΔEN if we don’t know the exact electronegativity of each atom? The answer is, you don’t. Most of the time, you will have to assume the ΔEN based on the periodic trends of electronegativity. But other times, the electronegativity chart will be given to you, just like the one below!
Ok, but what about molecular polarity? A molecule is polar when there is a separation of charge in the entire molecule. And don’t be surprised: bond polarity does NOT indicate molecular polarity. In other words, a molecule can have a polar bond but can still be non-polar. Why is that? To answer that question, we have to look at the 3-D shape of the molecule, known as the VSEPR theory. And that will be a topic for the next time! Polarity to be continued…
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