Covalent Bond Polarity: In a covalent bond, the shared electrons are not necessarily
shared evenly. If there are two identical atoms in a covalent bond, such as in hydrogen
gas, nitrogen gas, or oxygen gas, then the sharing of electrons is even. If, however, the
atoms are not identical, then the shared electrons will cluster more towards the atom with
the greater nuclear charge. For example, we have hydrochloric acid, HCl. In this
compound, the electron shared by hydrogen tends to be closer to the chlorine atom; also
the electron shared by the chlorine atom doesn't stray too far from the chlorine atom
itself. Thus, on the side of the molecule with the hydrogen, there's a slight positive
charge and on the side of the molecule with the negative charge, there's an equally slight
negative charge. This creates what is called an "electric dipole". An electric dipole is a
single system in which one side is positively charged and the other side is negatively
charged (these charges don't have to be equal in magnitude, but they almost always are in
natural systems).
Different atoms in polar molecules tug on electrons differently. For instance, in HCl the
shared electrons aren't as tightly clustered around the non-hydrogen atom as they are in
HF, hydrofluoric acid. Since different atoms tug on these shared electrons differently, a
measurable quantity called "electronegativity" has been devised. In Figure 17-22, the
electronegativities of all the elements are shown. Only the noble gases have zero
electronegativity--they basically don't pull on electrons in order to bond with them. We
see that the highest electronegativities tend to lie in the halogen group and in the other
columns on the right-hand side of the periodic table. In determining the strength of the
dipole of a molecule, it's the difference in electronegativity between the two molecules
that determines how polarized the molecule is. The greater the difference, the more
strongly attracted the covalent electrons are attracted to the atom with the greater
electronegativity and the greater the electric polarity of the molecule.
We can see from the table that ionic bonds tend to have the greatest polarity. For
instance, with francium fluoride, the difference in electronegativity is 3.98 - 0.70 = 3.28;
francium fluoride is the most ionic of the ionic bonds. Compare that with, say, lithium
astatide, LiAt: the difference in electronegativities is 2.20 - 0.98 = 1.22. Lithium astatide
is among the least ionic of the ionic solids. This means that the lithium has completely
given up its electron to astatine. Instead, the electron does hang around lithium a little
bit. There's no such thing as a perfect ionic compound. Some are just closer than others.
Smaller differences inherently occur in covalent bonds which, if you'll recall, can occur
only among non-metals. Restricting ourselves to the non-metals, we find that the largest
covalent dipole occurs between phosphorus and fluorine: 3.98 - 2.19 = 1.79. The
smallest difference occurs between two identical atoms bonding covalently. Between
different elements, the smallest covalent bond occurs with hydrogen and astatine, where
the difference is virtually zero. We will find that the polarity in the molecules plays a big
role in the chemical behavior of the molecule.
Molecular Polarity: For non-dipole molecules, determination of the polarity of the
molecule is most easily determined through the Electron Dot Structures. If we look at
water, we see an oxygen and two hydrogen molecules. The Electron Dot Structure of
water looks something like
Note the existence of two pairs of electrons from the oxygen atom that aren’t associated
with any hydrogen atom. Electrons can coexist in pairs despite their mutual electrical
repulsion because of magnetic effects. Each pair of electrons, though, will try to place as
much distance between themselves as any other pair of electrons. The configuration of
electron pairs that allows them to do this is a pyramidal structure or tetrahedron.