I don't understand the difference between a polar molecule and a polar bond. Please explain what the difference between those two is.
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2 Answers
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A polar bond is one where the charge distribution between the two atoms in the bond is unequal. A polar molecule is one where the charge distribution around the molecule is not symmetric. It results from having polar bonds and also a molecular structure where the bond polarities do not cancel. For example, water has polar bonds and its bent geometry means that there is a non-zero dipole moment. On the other hand, carbon dioxide has two polar carbon–oxygen bonds, but they are such that the polarities cancel out, so there is zero dipole moment.
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$\begingroup$ But even those non-polar molecules like CO2 do have a transitional dipole moment when distorted by vibrations such that the individual dipole contributions from each bond do no more cancel out. This is the reason why we can even observe a greenhouse effect with carbon dioxide: in order for the molecule to be able to absorb infrared radiation at a certain frequency it requires to have an oscillating net dipole vector upon distortion (e.g. bending away from linear to angled geometry and back). $\endgroup$ Commented Dec 2, 2016 at 10:38
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$\begingroup$ @user1573870 No argument there. On time average though, there is no net dipole. Based on OP's question, I don't we need to worry about those factors at the moment. $\endgroup$– ZheCommented Dec 2, 2016 at 14:39
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$\begingroup$ I agree this is an advanced topic, but it's relevant nevertheless since the question was also about polar /bonds/. E.g. we wouldn't have such an oscillating dipole e.g. in diatomic molecules consisting of only one type of element (N2, O2…), whose bonds are non-polar. $\endgroup$ Commented Dec 5, 2016 at 10:53
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1$\begingroup$ I would also amend that chemists use 'polar' synonymous with dipolar; other disciplines might consider any molecule with any kind of x-polar moment as polar. For example carbon dioxide is a quadrupolar molecule. In general, that way is more accurate in terms of explaining reactivity, but in general, the used terminology has served us well for a long time already. $\endgroup$– Martin - マーチン ♦Commented Nov 8, 2018 at 14:48
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Just because a bond is polar doesn't mean the molecule will be
All bonds where the atoms in the bond are not the same will be polar (in the sense that the electrons will be unevenly distributed between the two atoms resulting in a dipole). Some bonds (like C-H bonds won't be very polar, but they will have some polarity.
Molecules, though, are not polar just because they contain polar bonds. carbon tetrachloride ($\ce {CCl4}$) is a non-polar molecule that contains 4 polar bonds. But the polar bonds have dipoles that are symmetrically arranged around the carbon as a perfect tetrahedron and those dipoles therefore cancel each other out leaving the molecule with no net dipole moment. Chloroform ($\ce{CHCl3}$) where one of the chlorines has been replaced by a hydrogen, breaking the symmetry, is a fairly polar molecule. Likewise carbon dioxide is non-polar as the structure is linear and therefore the dipoles in the two polar bonds cancel. Carbon monoxide also has a polar C-O bond but there is no second oxygen to cancel out the dipole so it is polar.
So for molecules to be polar they need to contains bonds that are polar but they also need to be asymmetric enough for the bond dipoles not to cancel each other out.
answered Nov 8, 2018 at 21:45