Poisonous Neighborhoods of the Periodic Table

Painting by author, but castle-outline is from the cover of a forgotten book.

In describing the periodic table’s regions of related elements in Periodic Kingdoms,  P.W. Atkins drops the dry tone of textbooks and reaches for a little imagery.

The kingdom is not an amorphous jumble of regions, but a closely organized state in which the character of one region is close to that of its neighbour. There are few sharp boundaries. Rather the landscape is largely characterized by transitions: savannah blends into gentle valleys which deepen into almost fathomless gorges.

But as imaginative as he gets, the regions are the same ones we normally encounter. It would have been preferable if he had discussed the realm of gold, mercury, thallium and lead whose unique color, liquid state, poisonous nature and role in car batteries, respectively, can only be explained with relativistic effects. (In lead, the effect is explained here.)

Today I noticed some regions that are rarely, if ever, mentioned elsewhere—the periodic table’s poisonous neighborhoods. In the graphic below, I included either poisonous compounds or the ions responsible for the toxic nature of the substance. If the element does not appear by itself, as in the case of elemental sulfur, then it is not poisonous. Thallium is another example. The neutral version of the element is not found naturally. If synthesized artificially, the neutral form quickly reacts to form its poisonous version. The list is not meant to be exhaustive, but I tried to stick to the nastiest examples.

graphic created by author

Ingestion of aqueous HCl or hydrochloric acid is rare. But there is a detailed documented case of a 61 year old woman who in  a suicide attempt drank 200 ml of a 30-33% solution. By the time she had reached the hospital, the acid had killed tissue from the mouth to the base of her stomach, and ten hours later, she died of multi-organ failure. The gaseous form of HCl is also highly poisonous. A half hour exposure  to only 500 mg per liter of air (ppm) is fatal.

Much smaller concentrations of the related hydrogen fluoride gas can irritate the eyes, nose, and respiratory tract. A concentration of as little as 50 ppm will be fatal as it will cause the heart to beat erratically and fluid to accumulate in the lungs.

The protection we have against hydrogen sulfide poisoning is that our noses are extremely sensitive to its rotten egg stench. Most people can detect only 0.00047 ppm, a level that is 850 000 smaller than the approximately 400 ppm lethal level. H2S kills like cyanide by taking oxygen’s place and binding to iron cytochromes of the mitochondria.

Phosphine, PH3, smells like a blend of garlic on top of decaying fish. In 2010 in Utah, a freak accident  involving phosphine killed two children. Aluminum phosphide (AlP) was applied too close to the house by a pest control company. The phosphine gas formed by the reaction between the compound and water and what was intended for rodents seeped through a crack in the foundation and found its way into the home. It also killed two French-Canadian tourists in Thailand in 2010, and they were not the only victims. Phosphine poisonings are not uncommon in Southeast Asia.


One of the most bizarre cases of arsenic poisoning may have occurred in 2007 in Peru where the Carancas meteorite possibly produced arsenic-containing gases such as arsine, AsH3. It could have been formed when the meteorite’s hot surface met an underground water supply tainted with arsenic ions. More than 600 villagers presumably became ill. But the reports may have been exaggerated, and the gases emanating from the groundwater may have been sulfurous in nature and not arsine.

In 2010, in Zamfara, Nigeria, as many as 400 children died from lead poisoning. The villagers had ignored the dying of ducks a year earlier. For Pb (lead), ducks are sentinels like canaries in a coal mine. In this case, a nearby gold mine was the source of the lead contamination. Lead poisons by interfering with gene-regulating proteins, molecules that turn on and shut off the expression of DNA. The metal achieves this by displacing other metals from the protein complexes.


Prussian blue is the antidote to thallium poisoning, which was carried out routinely by Saddam Hussein. In a cruel case after Hussein’s death, still in Iraq, someone  with an axe to grind placed the tasteless, odorless poison in a large cake. Two children were killed. Thirty others were saved but suffered hair loss and dermatitis, two classic symptoms of the poisoning.

Mercury is another problem associated with gold extraction and Hg-contamination has become an issue in places like Segovia, Columbia. All forms of the metal wreck cellular function by changing the 3D (tertiary and quaternary) structure of proteins and by binding with sulfhydryl and selenohydryl groups.

Finally, cadmium poisoning or the itai-itai disease(translates to “it hurts- it hurts”) softens the bones and causes kidney failure. Cadmium is known to increase oxidative stress by being a catalyst in the formation of reactive oxygen species. In 2010, 12 million Shrek glasses sold by McDonald’s were recalled due to contamination by cadmium. There was not enough of the metal to cause poisoning, but the glasses were recalled because long term exposure to small amounts of cadmium can lead to cancer.

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