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Dipole Molecules
name Karusha R.
status student
age 15
Question - What is necessary to produce a dipole in a molecule?
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Karusha,
Molecules have dipoles when their electrical charge distributions are not
symmetrically oriented in space.
Use the Newton search engine: Type in "dipole." You should see other
postings which will offer details to assist your understanding.
Regards,
ProfHoff
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Dear Karusha,
The question really should be "what is necessary for a molecule
to have a nonzero dipole moment," because most molecules have
a dipole moment. Only molecules which are perfectly and
completely symmetric about their center of mass have a zero
dipole moment. This usually only happens with small
molecules consisting of 3-7 atoms, although there are
of course some exceptions.
Symmetry can be tricky to determine because one
first must write down the correct Lewis dot structure and
then use the VSEPR rules to predict the structure. For
example, a symmetric linear molecule like O==C==O has zero dipole
moment, but a symmetric bent molecule like H--O--H has a large dipole
moment. An asymmetric linear molecule like S==O==C has a dipole
moment also...
This is a brief answer, but I hope it helps you to go back
to your textbooks and find out more information.
Best, prof. topper
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To have a permanent dipole moment, a molecule must have an asymmetric
distribution of electrons with respect to the framework of the positively
charged atomic nuclei. This happens when some atoms comprising the molecule
have a higher ( or lower) attraction for the electrons -- this usually means
the outer valence electrons.
If a molecule is symmetric with respect to the center of mass, it cannot
have a dipole moment, even if the nuclei have a different affinities for the
electrons, because the electrons are attracted equally in opposite
directions and the net distribution of electrons is zero. For example, F2,
CO2, CH4, etc. do not have permanent dipole moments. On the other hand,
because the atoms in a molecule vibrate, and thus change position with
respect to one another, molecules -- even the symmetric ones -- can and do
have transient dipole moments when asymmetric vibrations cause the atoms to
not be symmetrically distributed. And one more detail -- a polar molecule,
charged or uncharged -- when it is close to another non-polar molecule can
"induce" a dipole in the nearby non-polar molecule.
Vince Calder
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Hello Karusha :
A molecular dipole (more correctly known as a permanent electric
dipole moment) is created whenever the geometric center of the overall
positive charge on the molecule does not coincide with the geometric center
of the overall negative charge of the molecule. This can happen whenever the
molecule is not symmetric. A molecule can be asymmetric for three reasons :
(1) all of the atoms in the molecule are the same, but are in different
physical locations. As an example, n-octane and 2,2,4-trimethyl pentane both
have eight carbon atoms, each having hydrogen atoms attached to the carbons.
However, n-octane has the carbon atoms in a straight line and is therefore
symmetric. 2,2,4-trimethyl pentane has some of the carbon atoms attached to
one of the central carbon atoms, so that the molecule is no longer a
straight line. You can see pictures of these molecules by going to
www.chemfinder.com and typing in the names of the molecules.) (2) one or
more atoms in the molecule are dissimilar. As an example, n-octane has, as I
mentioned before, eight carbon atoms arranged in a straight line, with
hydrogen atoms attached to each carbon. 1-octanol, on the other hand, has
eight carbon atoms arranged in a straight line, but one of the hydrogen
atoms on the end carbon atom has a hydroxyl group -OH attached to it,
instead of just a hydrogen atom. (3) the molecule has a "bent" configuration
because of unbonded electrons. As an example, look at a picture of a water
molecule. The two hydrogen atoms are pushed to one side of the oxygen atom
because of the lone-pair of electrons on the oxygen atom.
Molecules which have a large dipole moment, like water, are
generally good conductors of electricity, and have relatively high boiling
points, because the dipoles of each molecule align themselves with the
dipoles of the surrounding atoms.
Thanks for your question.
Jim
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