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Salt experiment breaks known chemistry rules?

Cubic NaCl3/Organov

Road salt and table salt seem like pretty ordinary substances, but according to recent research, at 200,000 atmospheres, they can violate known rules of chemistry. From ScienceDaily:

“We found, at low pressures achievable in the lab, perfectly stable compounds that contradict the classical rules of chemistry. If you apply the rather modest pressure of 200,000 atmospheres — for comparison purposes, the pressure at the center of the Earth is 3.6 million atmospheres — everything we know from chemistry textbooks falls apart.”

Standard chemistry textbooks say that sodium and chlorine have very different electronegativities, and thus must form an ionic compound with a well-defined composition. Sodium’s charge is +1, chlorine’s charge is -1; sodium will give away an electron, chlorine wants to take an electron. According to chemistry texts and common sense, the only possible combination of these atoms in a compound is 1:1 — rock salt, or NaCl.

“We found crazy compounds that violate textbook rules — NaCl3, NaCl7, Na3Cl2, Na2Cl, and Na3Cl,” says Weiwei Zhang, the lead author and visiting scholar at the Oganov lab and Stony Brook’s Center for Materials by Design, directed by Oganov. “These compounds are thermodynamically stable and, once made, remain indefinitely; nothing will make them fall apart. Classical chemistry forbids their very existence. Classical chemistry also says atoms try to fulfill the octet rule — elements gain or lose electrons to attain an electron configuration of the nearest noble gas, with complete outer electron shells that make them very stable. Well, here that rule is not satisfied.”

Chemists, what make you of all this? Hype or help?

Note: The findings may be relevant to planetary sciences, where “high pressure phenomena abound.”

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3 Responses to Salt experiment breaks known chemistry rules?

  1. This could be important. It’s reported in a top journal.

  2. A good reminder that what we claim to “know” isn’t always accurate, even when dealing with science – real every day science, as opposed to historical science.

  3. There is nothing particularly shocking in this, and no need to rewrite chemistry textbooks for it. Under extreme circumstances, atoms, molecules, electrons etc. that would normally behave in one way may behave in another.

    Normally xenon, an inert gas, cannot combine with other elements, because it has a “complete” outer shell under the octet rule, but in the lab, under artificial conditions, xenon compounds have been made. This information is over 40 years old and is found in undergrad chemistry intro textbooks from the 1960s/1970s. But no one ever suggested that the octet rule needed to be scrapped because of this.

    Similarly, at normal temperatures and pressures the usual explanation of salt formation applies. The fact that non-standard combinations of chlorine and sodium can be achieved at unnatural (for earth) pressures of 200 atmospheres doesn’t disprove basic chemical theory. At the most, it extends and modifies the theory, while leaving the basic principles intact.

    I’m not saying the results are uninteresting or that we should ignore them; they doubtless have something to teach about electrons, orbitals, how atoms bind, etc. But the suggestion that our basic chemistry is all wrong due to results of this sort is indeed “hype” — and hype typical these days, as scientists court public attention — and public funding.

    There was a time when scientists did not stoop so low as to try to impress the public, or other scientists, with extreme claims. Unfortunately science has become politicized and many scientists exercise poor judgment, over-claiming all over the place regarding their discoveries. We’ve seen this regarding fossils, for example.

    The proper thing to say is that such combinations show that the classical “octet” theory of chemical bonding is not a *complete* explanation of chemical combinations. That would be the modest, cautious, truly scientific way of speaking about this discovery.

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