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Chemical Bonds III: Polar Covalent

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  • 0:06 Covalent and Ionic Bonds
  • 0:52 Polar and Nonpolar…
  • 2:43 Partial Charges
  • 4:30 Lesson Summary
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Lesson Transcript
Instructor: Ori Gold
Are you confused about how you can tell what kind of bond two atoms will form? This lesson will help you understand the difference between polar and nonpolar covalent bonds as well as how to predict how two atoms will interact.

Covalent and Ionic Bonds

There are different ways that atoms can deal with the instability of not having a full valence level. The electronegativity of atoms, or the ability of the atom to attract a pair of bonding electrons, can help us predict how atoms will deal with this problem. When two atoms have similar or low electronegativities, they are more likely to share electrons to create a full valence level. This sharing of electrons is called covalent bonding. Two atoms with very different electronegativities will exchange electrons with each other. In other words, one atom will give another electrons to another atom, ending up with two ions. These two ions then come together and form an ionic bond.

What happens when two atoms have electronegativities that are similar enough to form a covalent bond but different enough not to share the electrons equally?

Polar and Nonpolar Covalent Bonds

You can think of atoms forming covalent bonds as kids playing tug-of-war on a playground. Ryan and Eric are playing tug-of-war. If both boys are equally strong, the middle of the rope won't move very much toward either kid. It will stay in the center. Just the same, if both boys are equally weak, the rope won't move a whole lot, either. The same thing happens with atoms. If both atoms have similar enough electronegativities, they'll pull on the electrons in the center toward themselves equally. As a result, they'll form a nonpolar bond, which is a covalent bond in which the electrons are shared equally. This type of bond is very common, especially between two atoms of the same element, like hydrogen or oxygen. But what happens if Eric is weak and Ryan is strong? The middle of the rope will end up closer to Ryan then to Eric. Again, the same thing happens with atoms in covalent bonds.

If one atom is more electronegative than the other, it will pull harder on those shared electrons in the center, forming a polar bond. A polar bond is a covalent bond in which the electrons are shared unequally. You can think of a polar covalent bond as being partway between a nonpolar covalent bond and an ionic bond. The atoms don't share electrons equally like they would in a nonpolar bond but they don't exchange electrons like they would in an ionic bond, either.

But remember that electrons exist in electron clouds called 'orbitals.' These electron clouds are only predictions of where the electrons might be. If we were to draw electron clouds here, the shared valence electrons could be found anywhere within the clouds. But since the shared electrons want to stabilize as the valence shells in both atoms, they're probably going to spend more time in the middle between these atoms.

Partial Charges

What about the charges on these atoms? In nonpolar covalent bonds, the electrons are equally shared between the two atoms. This means neither atom is more or less charged than the other. Both sides of the molecule are neutral. That is, they have no charge.

Ionic bonds, on the other hand, involve exchanging electrons. This results in two charged ions; one positive and one negative. Since polar covalent bonds involve an unequal sharing of electrons, they will result in partial charges on each atom. The more electronegative atom will pull the shared electrons closer to it, so that side of the molecule will be slightly more negative than the other side. In terms of electron clouds, the shared electrons will spend more of their time in the cloud of the more electronegative atom, making it partially negative.

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