Local truth

New Salt Com­pounds Challenge the Foun­da­tion of Chemistry

The ti­tle is overblown (it de­pends on what you think the foun­da­tion is), but get a load of this:

“I think this work is the be­gin­ning of a rev­olu­tion in chem­istry,” Oganov says. “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.”

And here’s the philo­soph­i­cal bit:

“For a long time, this idea was haunt­ing me—when a chem­istry text­book says that a cer­tain com­pound is im­pos­si­ble, what does it re­ally mean, im­pos­si­ble? Be­cause I can, on the com­puter, place atoms in cer­tain po­si­tions and in cer­tain pro­por­tions. Then I can com­pute the en­ergy. ‘Im­pos­si­ble’ re­ally means that the en­ergy is go­ing to be high. So how high is it go­ing to be? And is there any way to bring that en­ergy down, and make these com­pounds sta­ble?”
To Oganov, im­pos­si­ble didn’t mean some­thing ab­solute. “The rules of chem­istry are not like math­e­mat­i­cal the­o­rems, which can­not be bro­ken,” he says. “The rules of chem­istry can be bro­ken, be­cause im­pos­si­ble only means ‘softly’ im­pos­si­ble! You just need to find con­di­tions where these rules no longer hold.”

The ob­vi­ous ex­am­ple of lo­cal truth is rel­a­tivis­tic effects be­ing pretty much in­visi­ble over the du­ra­tions and dis­tances that are nor­mal for peo­ple, but there’s also that the sur­face of the earth is near enough to flat for many hu­man pur­poses.

Any sug­ges­tions for other truths which could turn out to be lo­cal?