atoms have more affinity for electrons than do other atoms,
and this affinity is based upon the size of the atom and the
distance of its valence electrons from its positvely charged
nucleus. The measure of an atom's electron affinity is called
it's electronegativity. Some atoms are highly electronegative
and steal electrons from any atom it bonds to; other atoms
donate electrons to any atom that it bonds to.
a general rule of thumb, as you go up and to the right of
the periodic table, the electronegativity of an atom 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 a bond (fluorine, at
the top right, is the most electronegative element). As you
go down and to the left, you reach the 98-pound weakling elements
such as the metals and silicon (and carbon, to some extent)
the illustration below, carbon atoms are shown as "weaker"
than fluorine atoms at holding onto the bonding electrons
(important note: "atoms" not drawn to scale. Fluorine,
despite being heavier, is actually smaller than carbon, although
is much more of an electron pig).
hogs such as fluorine selfishly steal electron density
from less electronegative atoms like carbon, creating
a bond dipole
there is a difference in electronegativity between two bonding
atoms (a carbon bonded to fluorine for example), you get a
separation of charge, or a dipole moment. In the C-F bond
example, carbon has some of its electron density stolen from
it by the highly electronegative fluorine atom and becomes
partially positive charged, while fluorine, having stolen
the electron density, becomes partially negative charged.
You can use the dipole vector (shown below) to indicate the
direction of the dipole moment in a bond, with the head pointing
towards the direction of negative charge.
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. It's
a good idea to look at many different bonds and predict the
dipole moment of the bond using the dipole vector.