Element of the day: Thallium

A piece of pure thallium in a glass container. Exposure to aiR would discolor the element.

Discovery and Applications

The next element in this series is thallium. At atomic number 81, thallium is a toxic heavy metal. William Crookes and Claude-Auguste Lamy discovered thallium in 1861 through a chemical experimental technique called flame spectroscopy, where a flame is used to see the type of light the compound emits. Each element releases light at a unique wavelength, and thallium's characteristic color in the emission spectrum is green. This element was named after the Greek word thallos, which means green twig. 

As mentioned earlier, thallium is extremely toxic. Exposure to it causes widespread effects on the body and will typically result in a painful death. Thallium sulfate (Tl₂SO₄) is frequent called the "poisoner's poison" since it is colorless, tasteless, and difficult to detect in the body (more on this later). Most people will die after exposure to 15 milligrams of thallium per kilogram of body weight. This means that it would not even take 1 gram of thallium sulfate to kill a 130 pound (60 kilogram) individual! A single teaspoon of thallium sulfate has about 34 grams, enough to kill 34 healthy adults. Today, thallium is mostly used to make glass with a high refractive index and is also present in gamma radiation detection equipment.

Advertisement promoting the sale of thalium-containing rat poison.

The Poisoner's Poison

The key to poisoning someone and getting away with murder is to use a compound whose symptoms no one recognizes or can detect. For instance, arsenic was frequently used as a murder weapon, but given the high frequency it was used to kill people, its symptoms became widely known. A chemical test was then developed in 1836 that marked the beginning of the end for arsenic as a stealth poison. In contrast to arsenic, thallium remained obscure for a much longer period of time and even today it manages to confuses the police. They would have to suspect thallium poisoning as the cause of death before even thinking of testing it. The main reason for this is because the symptoms of thallium poisoning are similar to the symptoms of many other conditions. Some of these symptoms include vomiting, hair loss, delirium, blindness, and abdominal pain. Today, thallium poisoning is detected by shining light through the sufferer's urine. Any thallium in it will absorb green light of a specific wavelength.

In the mid-1800s, thallium became known as the "poisoner's poison" since it was easy to give and difficult to detect. During this time period, thallium sulfate (a highly toxic form of thallium) was extremely simple to purchase since it was used in rat poison and as an insecticide. Thallium sulfate is now banned in most countries, although some developing countries still use it as a pesticide.

Aunt Thally (a.k.a. Caroline Grills) who was convicted of four counts of murder and three counts of attempted murder. 

In the 1950s, Australia went through a phase termed the "thallium craze" where many cases of murder or attempted murder by thallium poison were reported. In September 1952, Yvonne Fletcher was charged and tried for the murders of her first husband and second husband by giving them thallium. This was reportedly the first known case of thallium poisoning in Australia. In 1953, Veronica Monty was tried for the attempted murder of her son-in-law who was treated for thallium poisoning in 1952. She was found not guilty, but committed suicide in 1955 by ingesting thallium. Another infamous case in 1953 involved Caroline Grills who was sentenced to life in prison after killing her stepmother and three of her in-laws. She had attempted to kill three other people by serving them thallium-infused tea. While in prison, she was known as "Aunt Thally" due to her reputation.