Thallium (Tl)

Thallium sulfate was called the poisoner's poison: colourless, tasteless, water-soluble, and producing symptoms — nerve pain, confusion, then hair loss about two weeks in — that look like a dozen ordinary illnesses. Agatha Christie described it accurately enough in The Pale Horse that a nurse who had read the book recognised a real case of thallium poisoning in a baby in 1977. It kills by impersonating potassium: the body's potassium channels wave it through and it then disrupts every process potassium is involved in.

Properties

Atomic number

81

Symbol

Tl

Standard atomic weight

204.38 u

Category

Post-transition metal

Group

13

Period

6

Block

p-block

Electron configuration

[Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹

Electrons per shell

2, 8, 18, 32, 18, 3

Melting point

577 K — 303.9 °C — 578.9 °F

Boiling point

1,746 K — 1472.8 °C — 2683.1 °F

Density

11.8 g/cm³

Electronegativity

1.62 (Pauling)

Oxidation states

+1, +3

Discovered

1861 by William Crookes

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹

Noble gas shorthand

[Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 18 · P: 3

What it is used for

• Thallium-201 for cardiac stress imaging
• Infrared optics and low-melting specialty glass
• High-temperature superconductor formulations
• Historically, rat poison and ant bait — now banned almost everywhere

Hazards

• Highly toxic by ingestion, inhalation and skin contact; the lethal dose is around 1 gram
• Cumulative, with a body half-life of several weeks
• Symptoms are delayed and non-specific, which is what made it a favoured poison

IUPAC publishes an interval, [204.382, 204.385]; 204.38 is the conventional value.

Neighbours in the table

Group 13

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Thallium sits in group 13 with boron, aluminum, gallium, indium, nihonium.