BNG_Polar and Nonpolar Covalent Bonds Lesson
Polar and Nonpolar Covalent Bonds
Let's talk more about the sharing of electrons in covalent bonds. Pretend that you have a little brother and you both want the play with the same toy. What does your mom tell you to do? Share! That sounds good in theory, but how will that work exactly? What if you are much stronger than your little brother and you want the toy very badly? What will happen when mom is not looking? . . . You will get that toy!
Atoms participating in a covalent bond are very much like the sharing described above. They both want the electrons (the toy). What if one of those atoms wants the electrons more than the other? Will the sharing be equal or will it be more like the uneven sharing between you and your little brother? Take a look at these models and see if you can interpret the answer. The first picture represents a covalent bond between two identical atoms. The next picture represents a covalent bond between different atoms. What do you notice about the electron clouds?
Hopefully you could see that in the first picture, the electrons are shared equally (as shown by the equal distribution of the electron clouds). In the second picture, the electrons are not shared equally. When the electrons are shared unequally, the density of the electron cloud is greater in the space surrounding the atom that "wants" the electrons more. This is called a polar bond. The term polar comes from the 'poles' (like in a magnet) that are created by the uneven distribution of the electrons. The atom that wants the electrons more will end up with a partial negative charge. The other atom will end up with a partial positive charge. The partial charges are represented by the Greek letter delta, δ. When the electrons are shared equally, the bond is called a nonpolar bond. Note that when we use the terms polar and nonpolar to describe a bond, it is implied that it is a covalent bond. Some texts, however, will still use the terms polar covalent and nonpolar covalent.
Electronegativity
Did you notice that we used the phrase "wants the electrons more" several times above? So, what do we need to do now? We need to define what this means in real chemistry terms. The electronegativity of an atom is what we use to determine which atom "wants" the electrons more. The electronegativity is a measure of the atom's attraction of electrons. Technically, it is the ability of an atom in a molecule to attract a shared electron pair to itself.
Electronegativity generally increases up and to the right on the periodic table. Metals are the least electronegative of the elements. This means that they have very little ability to attract a shared pair of electrons. Remember that metals want to lose electrons, not gain (even by sharing).
Question: Try setting one atom to have a slightly higher electronegativity than the other. Where do the electrons end up?
Answer: closer to the one with higher electronegativity
Question: When the electrons end up shifted toward the nucleus of one of the atoms it causes the surface of the molecule around that atom to be:
Predicting Bond Type
While the electronegativity of an atom tells us how much attraction that atom has for electrons, it is the difference in the electronegativities of two elements that tell us what type of bond will form.
If the difference in electronegativities is none or very small (0.0 to 0.3), that bond is said to be a nonpolar covalent bond. The atoms share those electrons equally, and that bond has no partial charge.
If the difference is medium (between 0.3 and 1.7), the bond is considered to be polar covalent. Another way to say this is that the bond exhibits some ionic character.
If the difference is large (greater than 1.7), the bond is considered to be ionic.
Without the chart of electronegativities, you can still predict the bond type based on difference in position on the periodic table.
Remember to work on the module practice problems as you complete each section of content.
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