Inert Gases and Electron Shell Population
Date: September 2007
I was told that the inert gases are unreactive because
they are stable i.e. they have 8 electrons in their outer shell. But
that is not true for all of the inert gases, e.g. krypton. How do
you explain this?
Wow! Quite a question for a 6-8 grader!
So, I am going to give you quite an answer...
For many, many years people believed that
all of the noble gases were unreactive.
However, during the last part of the 20th
century it became clear that Xe and Kr could
participate in a few chemical reactions
(Neil Bartlett at U. British Columbia showed
that Xe could react with PtF6 in 1962).
So, although they are relatively unreactive
compared to other elements, they are not nonreactive.
As a result they are now called the *noble gases," and
they are no longer called the rare gases or the
inert gases in the scientific literature.
The stability of 8 electrons in an outer
shell is not a law of nature.
It is the Octet *Rule*, right?
A "rule" is a summary of many observations that
works for certain well-defined situations but
does not work for all cases.
Only the following elements
strictly obey the Octet Rule as far as
I am aware: C,N,O,F,Si,Ne,Ar. Other elements
may often obey it but there are exceptions.
For example, Br, P, S, Cl and I (iodine) can have
more than 8 electrons in their Lewis Dot
structures in certain compounds. B can have either
6 or 8.
Hope this helps.
The ground state electron configuration of:
bromine [Ar] +3d10, 4s2, 4p5;
krypton [Ar] + 3d10, 4s2, 4p6;
rubidium [Ar] + 3d10, 4s2, 4p6, 5s1
Your interpretation of the condition that "they have 8 electrons
in the outer shell" is a bit too constrained. The term "outer shell"
requires a somewhat broader definition. Imbedded in the definition is
an understanding that the "ns" and "np" orbitals have lower energy
than the "(n-1)d" orbitals, as you see from the series above. In the
case of rubidium, and following elements, the "5s" and "5p" orbitals
fill before the "5d". The source of your criterion (book/teacher) is
not as specific as it (could/should) be. You are commended for
observing the unstated condition on the Aufbau rules.
Unfortunately what you have been taught thus far is a simplification.
It is based on the general observation that the noble gases are inert
to chemical reactions, and the fact that we note that the general
electronic configuration of the most energetic orbitals of the noble
gases are such that the highest principal quantum numbered orbital has
its full complement of electrons. We, therefore, surmise that chemical
reactivity has everything to do with being able to fill-in these highest
principal quantum number. This is still true for Krypton which has an
electronic configuration of [Ar] 4s2 3d10 4p6. As such the highest
principal quantum number is 4 and the most energetic orbital "p" has
its full complement of 6.
A more detailed explanation of the Schroedinger Model of the Atom and
Molecular Orbital Theory gives a better rational to the inert nature
of the noble gases (as well as the existence of covalent bonds) but
this would be beyond the scope of this short explanation.
Greg (Roberto Gregorius
They do have a filled outer shell.
What is less clear is that the "outer shell" is just 8 electrons.
The 10 "d" electrons of the preceding transition-metal range might be
considered part of that outer shell.
However my CRC book shows Krypton's shell occupation to be,
from inside out, 2-8-18-8. that looks like an outer 8.
By the way, all these electron-orbit concepts do seems to have limits.
For the bigger inert gas atoms like Krypton, Xenon, and Radon,
the outer shell electrons are not totally impregnable,
they can be pried up and shared as a covalent bond by greedy halogens like
Fluorine or Chlorine.
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Update: June 2012