1
$\begingroup$

The Wikipedia article on electron shells states this, which I (a chemical layman) also always assumed:

The electrons in the outermost occupied shell (or shells) determine the chemical properties of the atom; it is called the valence shell.

It appears this is wrong. What really appears to determine the chemical properties is the outermost occupied subshell, ie. the things called 2s, 3p, etc. (Let's call this the "valence subshell" for now).

In particular, the noble gasses don't have a full valence shell at all (except helium and neon), but they do have a full valence subshell.

The same article also states that

A nonmetal atom tends to attract additional valence electrons to attain a full valence shell.

By this logic, noble gasses other than helium should be reactive.

I suspect this isn't simply a screw-up on Wikipedia's part, since the term "valence shell" is popular, while even the term "subshell" itself is rarely used. "Valence subshell" does return google results, but extremely few.

So, what's going on here?

$\endgroup$
  • $\begingroup$ You say: "By this logic, noble gasses other than helium should be reactive." How many electrons needs, in your opinion, the Neon atom, to obtain a full valence shell? One electron? Two electrons? How many? $\endgroup$ – Sofia Dec 12 '14 at 23:00
  • $\begingroup$ I think you are mostly hung up on semantics. The detailed reality of chemical bonds is far more complicated than what can be described with words like "valence shell" and it requires complex quantum mechanical calculations. Even the usual convention of introductory chemistry to treat the atomic bond problem as an extrapolation of the solutions of the single electron Schroedinger equation is essentially wrong in the general case. This oversimplification is necessary for purposes of teaching, because the real thing is far too complex to force on students early on. $\endgroup$ – CuriousOne Dec 12 '14 at 23:45
  • $\begingroup$ @Sofia You're right, Neon is an exception too. Argon however, has 10 electrons left to fill its third shell: The 10 electrons in the 3d subshell. $\endgroup$ – John Dec 13 '14 at 1:11
2
$\begingroup$

A couple of points.

outermost occupied subshell

generally everything that is in the outershell n highest (and sometimes electrons from lower subshells e.g. 3d in first row of transitions metals) has an impact on the chemistry and we can't generally say it is just the outer subshell.

noble gases

in general chemical discussion 8 electrons is considered a 'full' shell - and as pointed out by Sofia this is the case for Neon. You are, of course, correct that outer valence shells beyond Neon can take more than 8 electrons.

In line with Curious One's comment the situation is much more complicated than the description that you have looked at, but what can we do? At some point things have to be simplified for chemistry - a reasonable insight is that most non-metals head towards 8 electrons in the outer shell, though many times this rule is broken.

You might want to look at the difference between Valence Bond theory and Molecular Orbital theory - this is an old debate in chemistry as to how to describe 'bonding' in molecules. Neither is perfect and each has its merits and I don't plan to support one or the other here.

$\endgroup$
  • $\begingroup$ Thanks, also especially for this: "head towards 8 electrons". I never saw this spelled out anywhere. $\endgroup$ – John Dec 13 '14 at 1:25
  • $\begingroup$ @John Glad that this was useful. The 8 electron rule is good particularly for first row elements like C, N, O and where electrons are 'shared' in 'covalent bonds' e.g. water H$_2$O or ions where metals like Na loose an electron to become Na$^+$ and have eight electrons in the outer shell or non-metals like Cl gain an electron to become Cl$^-$ and again have eight electrons in the outer shell. $\endgroup$ – tom Dec 13 '14 at 1:53
  • $\begingroup$ Is there ever a case where the electrons taken on or passed away for covalent/ionic bonding at one time are from different subshells within the valence shell? If the answer to that was no, I'm really wondering why the focus isn't on the subshell only - or why "shell" is in fact not defined to mean subshell in the first place. $\endgroup$ – John Dec 13 '14 at 2:48
  • 1
    $\begingroup$ @John - Yes - take methane - CH$_4$ - C shares 4 electrons from the 4 hydrogen atoms, which each contribute one electron, plus the two 2s electrons and two 2p electrons from carbon. All these eight electrons are important for the bonding of the molecule. - there are many examples, which are similar to this. $\endgroup$ – tom Dec 13 '14 at 2:56
  • $\begingroup$ But even if both s and p electrons are used for bonding, it's purely p-"slots" getting filled at the carbon side and purely s-electrons lost by the hydrogen. Are there examples where s-"slots" and p-"slots" are both filled, or analogously s-electrons and p-electrons lost? I'm trying to establish if this "tendency to get whole" is really subshell rather than shell-based. $\endgroup$ – John Dec 13 '14 at 3:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.