MadSci Network: Physics
Query:

Re: Why don't noble gas ions easily form compounds?

Date: Fri Jun 10 18:11:01 2016
Posted By: Randall Scalise, Faculty, Physics
Area of science: Physics
ID: 1465513803.Ph
Message:

First, let's make sure that we are talking about the same concept.
Electron affinity is "the energy released ... when an additional
electron is attached to a neutral atom or molecule."
 http://goldbook.iupac.org/E01977.html

For helium and the other noble gases, the electron affinity is zero to
the precision of the measurements, not "massive".
 http://periodictable.com/Properties/A/ElectronAffinity.v.html

Next, there are a few mechanisms for binding atoms into compounds.
These are not mutually exclusive, so for example a bond could be
partially ionic and partially covalent.  Ionic bonds do not form
readily in inert gases because the ionization energy (the energy
required to remove an electron) is very high, 
 http://periodictable.com/Properties/A/IonizationEnergies.v.html

and as mentioned above the electron affinity is very close to zero.
The result is that it is not energetically favorable for the inert gas
atom to donate or to accept an electron, I will explain the reason
below.

Among the noble gases, the ionization energy is lowest for the heavier
atoms such as xenon.  This is because the outer electrons are
fractionally shielded from the nuclear charge by the inner electrons.
Heavy noble gases will form compounds with very electronegative
elements like fluorine: xenon difluoride (XeF2) and krypton difluoride
(KrF2).

Covalent bonds also do not occur readily in the inert gases.  Covalent
bonds are energetically favorable when the atoms participating in the 
bond can attain a filled outer shell of electrons by sharing electrons,
but the inert gases already have a filled outer shell, so they have 
nothing to gain.
 http://quatr.us/chemistry/atoms/covalent.htm

There is a very weak kind of binding called the van der Waals-London
interaction in which the noble gases do take part, but in order for
this weak force to stick atoms together they must be very cold
(otherwise thermal excitations will break the pairs of atoms apart).
Indeed, the noble gases other than helium will form solid crystals at
a few tens of kelvins.  These crystals though bound are not compounds,
however.

The question now might be: why are the electrons of noble gases bound
so tightly to their nuclei?  The answer lies in quantum mechanics.
Calculations in Hartree theory show that there is a large energy
difference between a completed p subshell and the next energy subshell
which is always an s shell with the next higher principle quantum
number.  But the average distance of the electron from the nucleus
increases when the principle quantum number increases, so the total
energy of the s electron is much higher.  This accounts for the large
ionization energies of the noble gases.  Also, because of their
spherical symmetry and paired electron magnetic dipole moments, there
is no external electric or magnetic field outside the noble gas atoms.
This makes it very difficult to interact with other atoms.
Colloquially put, there are no "handles" for other atoms to grab onto.

See sections 9-5 through 9-7 in the wonderful book "Quantum Physics of
Atoms, Molecules, Solids, Nuclei, and Particles" by Robert Eisberg and
Robert Resnick.

https://umadosedeinteligencia.files.wordpress.com/2014/09/quantum-physics-2nd-eisbergresnick.pdf


--Randall J. Scalise
 http://www.physics.smu.edu/scalise




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