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Bond Polarity

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Like the north and south poles of a bar magnet, Earth’s north and south magnetic poles—pictured above—are opposites in terms of their magnetic fields. Some types of chemical bonds and chemical compounds have “poles” similar to a bar magnet as well. But in the case of chemical bonds and compounds, the poles are opposites in terms of their electric charge. These bonds and compounds are described as polar.  

Polar and Nonpolar Covalent Bonds

Covalent bonds are chemical bonds between atoms of nonmetals that share valence electrons. In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar covalent bonds. The Figure below shows the polar bonds in a water molecule (H 2 O). The oxygen atom attracts the shared electrons more strongly than the hydrogen atoms do because the nucleus of the oxygen atom has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge, and the hydrogen atoms become slightly positive in charge. For another example of polar bonds, see the video at this URL: http://www.youtube.com/watch?v=1lnjg81daBs

In other covalent bonds, electrons are shared equally. These bonds are called nonpolar covalent bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral in charge. The oxygen (O 2 ) molecule in the Figure below has two nonpolar bonds. The two oxygen nuclei have an equal force of attraction for their four shared electrons.

Determing Polarity

The elements that are closest to haveing 8 valence electrons are will have high polarities. Florine is the most electronegative element on the periodic table with Francium is the least electronegative. A good rule of thumb when determining which atom in a compound is negative is to chose the element closest to Flourine. The element closest to chlorine will recieve the negative dipole and the other will recieve the positive dipole. Using the shape of the molecule and knowing the charges of the dipole you can determine the direction of the polarity. The direction of polarity goes in the direction of the negative dipole. 

Summary

  • In polar covalent bonds, electrons are not shared equally between the two atoms, so one atom is slightly negative in charge and one is slightly positive in charge. In nonpolar covalent bonds, electrons are shared equally so the atoms remain neutral in charge.
  • Covalent compounds with polar bonds may be polar or nonpolar, depending on their arrangement of atoms.

Vocabulary

  • polar : Having oppositely charged ends, as in polar covalent bond or polar covalent compound.
  • nonpolar : Not having oppositely charged ends, as in nonpolar covalent bond or nonpolar covalent compound.

Practice

Watch the video about polarity of molecules at the following URL, and then answer the questions below.

http://www.youtube.com/watch?v=LKAjTE7B2x0

  1. When does a polar covalent bond always produce a polar covalent compound?
  2. If a covalent compound has polar bonds and more than two atoms, what determines whether the compound is polar?
  3. Is water a polar compound? Why or why not?
  4. Which of the following compounds are polar?
    1. BF 3
    2. NH 3
    3. CCl 4
    4. CHCl 3

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