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Dipole-Dipole
Interactions
Some
atoms have more affinity for electrons than do other atoms,
based upon the size of the atom and the distance of its valence
electrons from the positvely charged nucleus. This results
in certain atoms (such as the halides, oxygen and nitrogen)
selfishly hogging electron density in chemical bonds with
carbon, and others donating electron density (such as metals).
As a general rule of thumb, as you go up and to the right
of the periodic table, the electronegativity, or the
measure of an atoms affinity for electrons, increases. Thus,
as you go up and to the right on the periodic table, you reach
the bully elements such as fluorine, oxygen, and nitrogen
who like to hog electrons. In the illustration below, carbon
atoms are shown as "weaker" than fluorine atoms
at pulling electron density from their chemical bond with
carbon (important note: "atoms" not drawn to scale).
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Electron
hogs such as fluorine selfishly steal electron
density from less electronegative atoms like
carbon
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If
there is a difference in electronegativity (a carbon attached
to fluorine for example), a permanent dipole is created on
a molecule. These dipoles, or separation of charge, allow
for interaction with other molecules by aligning opposing
charges with other molecules, and sticking to each other like
little atomic-scale magnets.

The
carbon-fluorine bond, shown above, has a dipole because fluorine
is a powerful electron withdrawing atom (with high electronegativity),
and carbon is not (low electronegativity). Thus, most of the
electron density of the bond resides on the fluorine, thereby
creating on it a partially negative charge . On the electron-deficient
carbon, the charge is partially positive. It is very important
to understand how electronegative atoms attached to carbons
form dipoles, as it is essential in the understanding of the
reactivity of organic molecules.
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