Thulium Unveiled: The Second Rarest Rare Earth in Periodic Videos

Overview

In this Periodic Videos episode Brady Haran and colleagues explore thulium, the second rarest rare earth, delving into its history, practical chemistry, and striking optical properties. The video highlights a fresh sample, simple acid reactions, UV-induced fluorescence, and a dramatic flame test that reveals a vivid emission.

Key insights

• Thulium is extremely scarce among the lanthanides, with only Promethean (promethium) rarer and radioactive, making thulium relatively accessible for demonstrations.
• Its salts fluoresce under ultraviolet light, a feature the team uses to visualize the thulium ions in solution.
• Reactions with weak acids yield observable precipitates such as thulium carbonate, and tests with sulfide and chromate reveal additional qualitative clues to its chemistry.
• The element’s name and symbol have an interesting history, with the ancient-inspired name Thuleum and a symbol change from Tu to tm.

Introduction

The video opens with a reminder that thulium is one of the rarest elements in the periodic table, ranking just behind promethium in abundance. The presenters explain that while thulium is a rare earth element, it is not radioactive and can be handled for educational demonstrations. A freshly obtained lump of thulium is shown, described as a high-quality sample from a collector who sent it in for analysis. The discussion soon moves to why thulium has captured interest in the Periodic Videos series, including how it behaves in everyday chemical tests and light-based experiments.

What is Thulium?

Thulium (Tm) sits in the lanthanide series and is part of the broader group of rare earth elements. The team notes its position in the periodic table and its unique electron configuration that gives rise to particular chemical and optical properties. The sample’s relatively low abundance is emphasized, alongside the fact that only promethium is rarer and radioactive, which limits practical use in demonstrations.

Discovery and Isolation

The narrative recounts the historical discovery of thulium in 1879 by the Swedish chemist Johan Gadolin’s contemporaries A. A. Klever or Cleave, though there is some ambiguity about the exact name due to language differences. The element was not isolated immediately because purification of rare earth elements is exceptionally tedious. The video notes that true isolation required extraordinary purification efforts, famously described as recrystallizing many thousands of times. The presenter's reflection on this labor-intensive history underscores the dedication of early chemists who pursued pure samples in spite of the complexity.

A New Sample and UV Fluorescence

A meaningful moment in the episode is when the presenters describe receiving a thulium sample through the post. This sample enables a Bunsen flame test and UV-driven fluorescence experiments. When illuminated with ultraviolet light, thulium salts absorb energy and re-emit it as visible light, producing a pale blue glow that Brady’s camera can detect. To maximize the fluorescence signal, they temporarily relocate to Neil’s office, which has no external windows, making the glow easier to observe under UV illumination.

Reactivity and Salt Chemistry

Back in the lab, the solution containing thulium is diluted with water and then subjected to several tests. In the first test, potassium carbonate is added, producing a white precipitate identified as thulium carbonate. A subsequent test with sodium sulfide also yields a white precipitate, accompanied by the characteristic smell of rotten eggs, underscoring the sensory aspects of chemical testing. A final test with potassium dichromate in the acidic medium yields a color change and a moderate precipitate, suggesting the possible formation of thulium chromate, a less common compound that the team speculates about observing for the first time in such demonstrations.

Flame Test and Spectacle

The episode reaches a dramatic moment when thulium is ground into very fine particles and sprinkled into the Bunsen flame. The burning particles emit a bright, greenish light and produce crackling noises, delivering a powerful visual demonstration of the element’s flame characteristics. The team concludes that thulium proves more intriguing than initially expected, offering a vivid combination of chemistry and spectroscopy in a compact demonstration package.

Naming and Symbol

The video explains the etymology of Thuleum, a name drawn from an ancient Greek reference to northern regions with Scandinavian connections. Early proposals suggested the symbol Tu for this element, but the symbol was ultimately changed to tm. The presenters offer a light-hearted suggestion that Tu may have conflicted with a French word, but they acknowledge there could be another reason for the change. The choice of symbol remains a nod to linguistic and historical considerations that accompany chemical nomenclature.

Concluding Thoughts

In wrapping up, the presenters reflect on how a seemingly quiet element can yield striking phenomena when engaged with carefully chosen tests and bright UV light. The thulium sample, the subtle acid reactivity, the fluorescence, and the dramatic flame emission collectively reinforce the idea that rare earth elements can be both scientifically informative and visually captivating. The video closes with thanks to supporters and a reminder that more chemistry content can be explored across the platform.