[Phys-l] chemical bonds

[John Denker <jsd@av8n.com>]

On 12/15/2010 04:10 PM, LaMontagne, Bob wrote:

> Hate to appear dumb - but what are "bonds"? Are they physical
> entities? Are they a euphemism for something else? Are they a
> scenario?

That is not a dumb question.  Reeeally not.

Bonds exist, and can be understood in terms of physics.  If you look 
at it in just the right way, there is a fairly simple picture that 
fits the facts *and* agrees with more-detailed more-difficult theory.

We've all been told that bonds are "made of" electrons ... but before
we swallow that idea, we should look at the data:
   N≡N  triple bond                 5 electrons per atom in N=2 shell
   O=O  allegedly double bond       6 electrons per atom in N=2 shell
   F-F  single bond                 7 electrons per atom in N=2 shell
  Ne Ne no bond                     8 electrons per atom in N=2 shell

So in this part of the periodic table, the trend is clear: 
      More electrons means less bonding.

We can understand this as soon as we realize that in this situation,
we are adding electrons to _antibonding_ orbitals, and these particular
electrons do nothing except try to blow the atom apart.  So when we
count bonds, we are not counting the presence of bonding electrons;
instead we are counting the absence of antibonding electrons.  It's a
double negative.

Equivalently, you can talk in terms of holes.  The word here means the
same thing it means in connection with P-type semiconductors.

They probably didn't tell you any of this in first-year chemistry class.

For a great deal more detail on this, including diagrams and many
additional examples, and even counterexamples, see
   http://www.av8n.com/physics/draw-molecules.htm

==========================

The other weird thing about bonds is that chemists more-or-less universally
speak in terms of the bond energy (or "heat of formation") as a positive 
number, even though it makes a negative contribution to the energy content 
of the sample.  Let's be clear:  A stronger bond means a more-negative (or 
less-positive) energy content.  This leads to confusion because in almost 
any other context people would talk about the _energy content_ directly (for 
instance, as we saw in connection with the Bernoulli equation).

This is genuine, non-hypothetical problem.  Very commonly people think they 
can break bonds and get out the energy, just like they break an egg to get 
out the contents.  This is diametrically wrong.

If you want a powerful fuel, you want the reactants to be weakly bonded
and the products to be much more strongly bonded.