81TlThallium204.38

A real sample of the element Thallium — Real sample photo

Thallium (Tl)

“Discovered by a green line in a flame, feared as the tasteless “poisoner’s poison” — now injected to photograph a struggling heart.”

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Atomic number81

Atomic mass204.38

Group13

Period6

BlockP-block

PhaseSolid

Electron structure

How its electrons are arranged

[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹

Shell 1

2e⁻

Shell 2

8e⁻

Shell 3

18e⁻

Shell 4

32e⁻

Shell 5

18e⁻

Shell 6

3e⁻

Computed from the aufbau principle, with known real-world exceptions applied automatically.

🎯 Exam focus

Questions to practise

Try each question yourself first — then open it to read the full model answer.

1How did a green line in a flame spectrum reveal thallium to William Crookes in 1861, and what advantage did spectroscopy hold over the chemical tests of his era?

Model answerCrookes was examining the gunky residue of a sulfuric-acid plant when a sample’s flame spectrum showed a brilliant green line that belonged to no known element — the fingerprint of a new metal, announced as thallium in 1861. Spectroscopy beat the chemical tests of his day because every element emits its own characteristic lines, still recognisable in traces far too small for wet analysis to detect or separate.

2Why can thallium-201 light up a struggling heart on a gamma camera — explain the potassium-mimicking chemical behaviour behind it.

Model answerInjected thallium-201 chloride dissolves as Tl⁺ ions, whose single positive charge so closely imitates potassium that the body treats them as potassium: healthy heart muscle, always hungry for potassium, absorbs the thallium through its channels. Well-perfused heart tissue therefore takes up the isotope and lights up the gamma-camera image, while under-supplied regions stay dark — the scan maps the heart’s blood supply.

3Name one hospital machine and one literary work where an ordinary person is likeliest to have met the word “thallium”.

Model answerIn hospital, the gamma camera of the nuclear-medicine wing, whose thallium-doped NaI(Tl) and CsI(Tl) scintillator crystals turn invisible gamma rays into countable flashes of light — the same doping serves airport baggage scanners. In literature, Agatha Christie’s 1961 thriller The Pale Horse, whose victims’ burning feet and shedding hair made “thallium” famous far beyond chemistry.

4Thallium shows both +1 and +3 oxidation states, yet Tl⁺ dominates its chemistry — name the effect responsible and explain it in a sentence.

Model answerThe effect is the inert pair effect. Thallium’s two 6s electrons are held so tightly that they refuse to take part in bonding, so the atom normally loses only its single 6p electron and settles as Tl⁺ — a stable +1 state whose salts behave almost like silver’s, while the +3 state familiar from boron and aluminium becomes the exception.

5Write thallium’s electron configuration, identify its group, period and block, and point out which electron is its differentiating one.

Model answerThallium’s configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹. The outermost occupied shell is n = 6, so it belongs to Period 6; the last electron enters a 6p orbital, making it a p-block element; and with 6s² 6p¹ it carries three valence electrons, which places it in Group 13. The differentiating electron is that lone 6p¹ electron — the eighty-first and last to arrive.

6Keeping in mind how Crookes first met this element, explain why “green shoot” is such a fitting souvenir in its name.

Model answerCrookes first met the element through the vivid green line in its flame spectrum, and the name keeps a souvenir of exactly that meeting: thallium comes from the Greek thallos, a fresh green shoot of spring. Just as no known element could claim the green line, no discovery story could suit the name better — an element revealed purely by green light is remembered forever as “green shoot”.

Quick answers

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Electron configurationThe full arrangement, step by step ValencyHow many bonds it forms Atomic massThe standard weight, explained Metal or nonmetalWhich family it belongs to Everyday usesWhere you meet it daily

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The notes

About the element

Characteristics

One of the most toxic of the common metals: odourless and nearly tasteless Tl⁺ ions hijack the body’s potassium channels, causing burning nerve pain and dramatic hair loss — signs easily mistaken for other diseases.
A split personality in group 13: like boron and aluminium it can show +3, but the inert pair effect makes the +1 state of Tl⁺ so stable that its salts behave almost like silver’s.
Physically lead-like — soft, dense (11.85 g/cm³), low-melting near 304 °C — yet unlike lead it melts obligingly in the flame of an ordinary spirit lamp.
It tarnishes quickly in moist air as it reacts with oxygen and water vapour, so laboratories store sticks of it under oil or in sealed ampoules, much like the alkali metals.

Appearance

A soft, bluish-grey metal soft enough to score with a fingernail and slice with a knife. A fresh cut gleams like newly cast lead, but within minutes in open air a dull grey oxide film creeps across the shine.

Real world

Where you meet it

✦ Uses

Nuclear cardiology: thallium-201 chloride is injected for heart stress tests — mimicking potassium, healthy heart muscle absorbs it and lights up the gamma-camera image of the blood supply.
Thallium-doped scintillator crystals — NaI(Tl) and CsI(Tl) — convert invisible gamma rays and X-rays into countable flashes of light inside hospital gamma cameras and security scanners.
Infrared optics: crystals of thallium bromide–iodide transmit long-wavelength infrared, serving thermal-imaging lenses, $\mathrm{CO_{2}}$ -laser windows and infrared spectrometers.
The former rodenticide thallium sulfate — banned in most countries once its human toll was tallied; it survives mainly in crime fiction and forensic textbooks as the classic “poisoner’s poison”.

◍ Everyday examples

A heart patient leaving the nuclear-medicine wing of a big city hospital may have just had a thallium-201 scan map the blood flow through the heart muscle.
Airport baggage scanners and hospital gamma cameras owe their light-flashing detector crystals to a pinch of thallium doping.
Crime-fiction shelves keep thallium famous: in The Pale Horse the victims’ burning feet and shedding hair are the textbook symptoms of its poisoning.

Story

History & name

In 1861 William Crookes, examining the gunky residue from a sulfuric-acid plant, spotted in its flame spectrum a brilliant green line belonging to no known element — and named the newcomer thallium, from the Greek thallos, a fresh green shoot of spring. Claude-Auguste Lamy in France isolated the metal itself the next year. Its tasteless sulfate became a notorious rat poison in the twentieth century — and Agatha Christie’s 1961 thriller The Pale Horse described thallium poisoning so accurately that readers have recognised real cases from its pages.

Name originGreek thallos, a green shoot — the souvenir of the vivid green spectral line that revealed the element.

Neighbours

Nearby elements

The elements that sit right beside this one on the table — often the closest chemical cousins.

79AuGold196.97 80HgMercury200.59 82PbLead207.2 83BiBismuth208.98