polar molecules by tomsgreathits


									Basic concepts: polar and non-polar molecules.
Chemists live in a world made up of atoms made up of protons, neutrons, and
             Protons (positive charge and some mass) and
             neutrons (which are just a squosh more massive than protons)
               hang out together in the nucleus of your atom, while
             electrons can be thought of as zipping around the nucleus.

When multiple atoms are part of an assembly in which they are bonded to each
other, you have a molecule. For the moment, consider the "bond" between
atoms in a molecule to be an electron-sharing arrangement that maintains a
certain (average) spatial configuration between the nuclei of the bonded atoms. [1]
To the extent that the bond hooking the
atoms together [2] is made of electrons, a
crucial matter is how equally, or
unequally, the bonded atoms are sharing
these electrons.

Some atoms "want" [3] electrons more than
others; trends in electron appetite (or
electronegativity) are among the many
trends which make it useful to keep a
Periodic Table of the elements around.

 The first thing to notice is that whenever
  you have identical atoms bonded to each
  other, the electron tug-of-war between
  them is a tie.
 In a molecule of H2, the two hydrogens are
  equally matched; each pulls electrons just
  as strongly as the other.
 Thus, the electrons in the bond are equally
  shared between the two atoms, and the
  bonding is covalent.
 The same will be true of Cl2, Br2, I2, O2, N2, and any other diatomic
  homonuclear molecule (i.e., two-atom molecule with one type of atom).

In situations like these, where the electrons in the bond are shared equally, the
bonds are non-polar. This means that no one end of the bond has more or
less electron density than the other end.
Not all bonding situations are so egalitarian, however.
If you bring together two atoms with very different electronegativities, they will not
form a bond with any semblance of electron sharing at all.

For example, the chlorine atom would
sell its own mother[4] to pick up an extra
electron of its very own, while the
sodium atom would like nothing better
than to get rid of one of its electrons.
This is why NaCl has no covalent
bonds to speak of -- Cl rips off an
electron from Na to leave us with Na+
and Cl- ions that hang out together due
to electrostatic attractions between the two charged ions.

The electrostatic association between Na+ and Cl- ions is often called ionic
bonding, even though there's no "bond" in the sense of shared electrons between
the atomic nuclei. Instead of electron sharing here, it's a "winner take all" situation.

Not every situation that brings together two atoms with different electronegativities
results in so lopsided a match.

 For example, in a molecule of hydrofluoric
acid, the difference in electronegativities
between H and F is not so large that F walks
away with all the electrons.
 However, F pulls the electrons from the
   bond more strongly than H does, so that
   the sharing of electrons between these
   two nuclei is not equal
 As a result, the electrons in the bond
   spend more of their time hanging out near F than they do hanging out near H.

When the electrons in the bond are shared, but they aren't shared equally, what
you have is a polar covalent bond.
In hydrofluoric acid, the polar covalent
bond means that the electron density is
unequally spread over the whole molecule.

 There's one end of the molecule (the F-
  end) with a higher electron density, while
  the other end of the molecule (the H-end)
  has a lower electron density.
 Note that HF is a linear molecule, so it
  only has two ends.

 The end of the molecule with a higher
  electron density has a partial negative
  charge, while the end with a lower
  electron density has a partial positive charge.

 This uneven spread of electron density is what makes HF a polar molecule.
 When molecules have positive ends and negative ends, this has some
  consequences for how they will interact with other molecules.

 For example,
 if you have a whole bunch of polar
   molecules like HF close to each other,
   they will tend to be arranged in ways
   that maximize the attractions between
   opposite charges and minimize the
   repulsions between like charges.

    In other words, the positive end of one
    molecule will tend to hang out with the
    negative end of another molecule, and so on.

    (Note that my cartoon version of the happy arrangement of a bunch of HF-like
    polar molecules is only including two dimensions. Real molecules interact with
    each other in three dimensions. [5])
While polar bonds are a necessary ingredient of a polar molecule, they are not a
sufficient condition for polar molecules.

Example, In carbon tetrachloride, the
difference in electronegativity between C
and Cl means that each of the four C-Cl
bonds is polar, with more electron
density being pulled toward the Cl and
away from the C.

However, the CCl4 molecule has a
tetrahedral shape, which means that
the partial negative charges on the Cl
atoms are distrinuted pretty
symmetrically around the molecule.

Meanwhile, the partial positive charge
on the C is buried in the center of the molecule. Thus, the CCl4 molecule doesn't
have a positive end or a negative end -- instead, despite those nice polar bonds,
it's a non-polar molecule.

Of course, one of the most famous polar molecules of all time is water, H2O.

 In this chemist's cartoon of a water
  molecule, those things that look like
  Mickey Mouse ears on the O atom
  are lone pairs of electrons -- pairs
  of electrons that are associated with
  the O atom but that are not involved
  in bonding.
 Oxygen pulls the bond electrons
  with more zeal than do the
  hydrogen atoms, so there's a
  delocalization of those electrons
  away from the H atoms and toward
  the O atom.
 This results in a bent molecule
  shaped almost like a delta kite, with
  a partial negative charge on the pointy O nose of the kite, and partial
  positive charges on the H corners at the tips of the wings.
Why is the H2O molecule bent rather than linear? It's those two lone pairs of
electrons sitting on the O atom. They are greedy for space in order to keep their
distance from other sources of negative charge on the O (like charges repel and
all that). Those other negative bits include the electrons in the H-O bonds. So H2O
has an arrangement that's pretty similar to that of CCl4 -- a central atom with four
electron-packed thingies arranged around it, each of those thingies needing to be
as far from the others as possible. In H2O, it's just that two of those thingies are
lone pairs of electrons.

The fact that water, as a polar molecule, looks more like a delta kite than a bar
magnet means that arranging a bunch of water molecules in close proximity to
each other is a wee bit more complicated.

 The general idea is the same:
 you want to put negative ends near positive ends and positive ends near
   negative ends.
 Here, though, you have two positive H ends per molecule. Luckily, the partial
   negative charge on the O atom is big enough that it can hang out with two
   positive H ends from neighboring water molecules.
 This results in the chummy intermolecular association represented in the
   cartoon below

Understanding the difference between polar and non-polar molecules makes it
easier to understand solubility (and insolubility). As well, it's a useful concept to
have under your belt if you want to soothe a cranky baby or entertain your
sweetie with a discussion of intermolecular forces.

To top