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 (H2O). 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 (O2) molecule in the Figure below has two nonpolar bonds. The two oxygen nuclei have an equal force of attraction for their four shared electrons.
Polar and Nonpolar Covalent Compounds
A covalent compound is a compound in which atoms are held together by covalent bonds. If the covalent bonds are polar, then the covalent compound as a whole may be polar. A polar covalent compound is one in which there is a slight difference in electric charge between opposite sides of the molecule. All polar compounds contain polar bonds. But having polar bonds does not necessarily result in a polar compound. It depends on how the atoms are arranged. This is illustrated in the Figure below. In both molecules, the oxygen atoms attract electrons more strongly than the carbon or hydrogen atoms do, so both molecules have polar bonds. However, only formaldehyde is a polar compound. Carbon dioxide is nonpolar.
Q: Why is carbon dioxide nonpolar?
A: The symmetrical arrangement of atoms in carbon dioxide results in opposites sides of the molecule having the same charge.
- 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.
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.
Watch the video about polarity of molecules at the following URL, and then answer the questions below.
- When does a polar covalent bond always produce a polar covalent compound?
- If a covalent compound has polar bonds and more than two atoms, what determines whether the compound is polar?
- Is water a polar compound? Why or why not?
- Which of the following compounds are polar?
- What are polar covalent bonds? Give an example.
- Why are the covalent bonds in an oxygen molecule (O2) nonpolar?
- Carbon (C) atoms attract electrons a little more strongly than hydrogen (H) atoms do. The models below represent three covalent compounds containing only carbon and hydrogen atoms. Are the compounds polar or nonpolar? Explain.