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Status: student
Grade: 6-8
Location: NY
Country: USA
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?

Dear Philip,

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.
-Dr Topper

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.

Vince Calder


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.

Jim Swenson

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