The textbook rules of chemistry are unassailable, unless you have the “Devil may care” attitude of sodium chloride, otherwise known as table salt. According to a study at DESY’s X-ray source PETRA III and at other research centers, salt produces “forbidden” (according to the rules of chemistry) compounds when placed under high-pressure experiments.

Salt is one of the most studied and stable chemical compounds. Its chemical composition is simple – one sodium atom (Na) and one chlorine atom (Cl). At least, that’s the case under benign, everyday conditions. Other compounds of the two elements break the written rules of chemistry. For instance, according to the octet rule all chemical elements strive to fill their outermost shell with eight electrons, which is the most stable configuration, found in noble gases. Sodium has one extra electron and chlorine is missing one, so sodium donates one electron to chlorine, leaving both atoms with an outer shell containing eight electrons and forming a strong ionic bond.

When scientists place the salt under the extremely high pressure of 200,000+ atmospheres and pepper (ha) in an extra pinch of either sodium or chlorine, things get even weirder. “Following the theoretical prediction, we heated the samples under pressure with lasers for a while,” explains co-author Dr. Zuzana Konôpková of DESY, who supported the experiments at DESY’s Extreme Conditions Beamline P02 (ECB). “We found other stable compounds of Na and Cl which came as a surprise.”

Those “other” compounds include Na3Cl and NaCl3, and they should not exist. Those chemical bonds require much more energy to remain stable, and nature, being the cruel efficient mistress that she is, always defaults to the lowest energy state.

Previous calculations hypothesized that these and other “exotic” compounds could form and even remain stable under the extreme conditions of the experiment. Scientists tested the predictions in what they call “cook and look” experiments, targeting Na3Cl and NaCl3, the two compounds that were predicted to be more easily made than others, and they indeed found them.

“We have predicted and made crazy compounds that violate textbook rules: NaCl3, NaCl7, Na3Cl2, Na2Cl, and Na3Cl,” says Dr. Weiwei Zhang, the lead author of the paper and a visiting scholar at Oganov’s lab at Stony Brook.

“These compounds are thermodynamically stable and once made, remain so indefinitely,” says Zhang. “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.”

One of the reasons the scientists were able to produce the “forbidden” compounds is that the rules of chemistry really only apply under what chemists call “ambient conditions.” Though the default for us on Earth, our conditions are actually rather unique on a universal scale.

“The rules of chemistry are not like mathematical theorems, which cannot be broken. The rules of chemistry can be broken, because impossible means softly impossible. You just need to find the conditions where the energy balance shifts and the rules hold no more” said Oganov.

The research has implications that could help other researchers learn more about planetary cores, as well as lead chemists to developing new, practical compounds.

Salt: The rule-breaker your mother always warned you about.