Re: If two atoms approach each other shouldn't they experience a ?force of repulsion?

In article <5929c37f-d03c-40ba-b414-f5c2f66174aa@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, uri <danny99@xxxxxxxxxxxx> wrote:
: On Jan 28, 3:39 am, Fred Kasner <fkas...@xxxxxxxxxxxxx> wrote:

:> Actual collision is not needed. Close approach with long enough
:> proximity allows electronic interactions that can lead to bond formation.

: But why do atoms form chemical bonds? Why does close proximity allow
: electronic interactions that lead to bond formation?

In the (not unreasonable) approximation that electrons are found in
orbitals, when two orbitals of the proper symmetry interact, two new
"molecular" orbitals are formed, one of which is lower in energy than
the atomic orbitals (due to constructive interference between them) and
the other is higher in energy than the atomic orbitals (due to destructive
interference between them). If the total energy of placing all of the
electrons in molecular orbitals is lower than that of placing them in
atomic orbitals of the separated atoms, then the system is more stable if
the two atoms are bound; otherwise, the system is more stable if the atoms
remain unbound.

The usual example that is given as a start is two atoms that only have
a single orbital each (i.e. hydrogen-like or helium-like atoms). Consider
two H atoms. Each has one electron in a 1s orbital. As the atoms
approach, the orbitals interact to form two molecular orbitals, which
are called (due to the symmetry, for reasons that need not concern us here)
"sigma" and "sigma-star." (The actual Greek letters and an actual asterisk
are used.) The sigma orbital is the sum of the two atomic orbitals, and
is lower in energy than either of them, while the sigma-star is the difference
between the atomic orbitals, and is higher in energy.

According to the Pauli Exclusion Principle, we can only place a maximum of
two electrons in any single orbital. In the case of two hydrogen atoms,
we only have two electrons total, so they both will go in the sigma
orbital, and the system will be more stable if the two atoms are bound,
forming the H2 molecule. Because placing electrons in the sigma orbital
*stabilizes* the system, it is called a bonding orbital.

Because this stabilizing force is opposed by the repulsion of the two
nuclei, there will be an H-H distance at which the total energy reaches a
minimum, so the equilibrium distance between the two atoms will be where
the total energy is minimized.

Now consider two He atoms. In this case, there are a total of four electrons.
If we put two in the sigma orbital and two in the sigma-star orbital, the
total energy of the system does not decrease, since the stabilization
caused by putting two electrons in the sigma orbital is opposed by the
destabilization caused by putting two electrons in the sigma-star orbital;
an orbital that *destabilizes* a system when electrons are put in it is
thus called an "anti-bonding" orbital. In some cases, putting electrons in
a particular molecular orbital has essentially no effect on the overall
energy; these orbitals are called "non-bonding" orbitals. So we can see
that in the case of He2, there is no net stabilization if the two atoms
are bound together, so no bond is formed.

This is, of course, an oversimplification (what I've told you so far cannot
explain why HeH is not stable, but (He2)+ is, for example), but it illustrates
the basic principle.

-----
Richard Schultz schultr@xxxxxxxxxxxxxx
Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Opinions expressed are mine alone, and not those of Bar-Ilan University
-----
"Contrariwise," continued Tweedledee, "if it was so, it might be, and
if it were so, it would be; but as it isn't, it ain't. That's logic."

.

Relevant Pages

  • Re: speed, size, time
    ... That's where my own shrinking atoms came from, ... then Dark Energy, stuff you cannot encounter for some hundreds or thousands ... Conduction bands are the electrons ... >>Electrons in orbitals, are different than electrons in conduction bands. ...
    (sci.physics.relativity)
  • Re: Upended quantum physics in the news
    ... some iron oxides but not others, and chromium dioxide but not atomic ... Iron has 26 electrons. ... M shell the 3s and 3p orbitals are also completely filled ... For atoms that only fill up to the 3d orbital, ...
    (talk.origins)
  • Re: Question from Eisberg and Resnick.
    ... > in the quantum numbers of the electrons, but frankly I do not really ... atoms in an infinite lattice combines the orbitals into "bands". ... These are the "band gaps". ...
    (sci.physics)
  • Re: My New Website
    ... > the number of electrons, i.e. experiments which would have given ... a very abstract claim that i dont have to believe you ... > that Moseley's law does not work for heavy atoms. ...
    (sci.physics)
  • Re: Gravity and Magnetism
    ... >> process is essentially the same, except that you now have electrons ... >> I'll try to describe what happens in the filament. ... >> What happens here is that the positively ionized bunch of atoms ... At the border of the positive zone, one of the atoms of the ...
    (sci.physics.relativity)