Thallium - Tl

Atomic number

81

Atomic mass

204.383 g.mol ^-1

Electronegativity according to Pauling

1.8

Density

11.71 g.cm^-3 at 20°C

Melting point

304 °C

Boiling point

1473 °C

Vanderwaals radius

0.182 nm

Ionic radius

0.099 nm

Isotopes

11

Electronic shell

[ Xe ] 4f¹⁴ 5d¹⁰ 6s^{2 6p1}

Energy of first ionisation

589.1 kJ.mol ^-1

Energy of second ionisation

1970.5 kJ.mol ^-1

Energy of third ionisation

2877.4 kJ.mol ^-1

Discovered by

Sir William Crookes in 1861

The element and its compounds are toxic and should be handled carefully.

Thallium occurs in the environment naturally in small amounts. It is not applied very widely by humans, merely as rat poison and as a substance in electro-technical and chemical industries. These applications can cause human exposure to thallium substances.

The human body absorbs thallium very effectively, especially through the skin, the breathing organs and the digestive tract.

Thallium poisoning is mainly caused by accidental uptake of rat poison, which contains large amounts of thallium sulphate. Consequently, stomachaches will appear and the nervous system will be damaged. In some cases the damage is so irreversible that death will soon follow. When a human survives thallium poisoning often consequences of disturbances of the nervous system, such as trembling, paralyses and behavioural changes will remain. With unborn children thallium poisoning can cause congenital disorders.

Due to accumulation of thallium in the bodies of humans, chronic effects consist, such as tiredness, headaches, depressions, lack of appetite, leg pains, hair loss and disturbances of the sight.

Further effects that can be related to thallium poisoning are nerve pains and joint pains. These are consequences of thallium uptake through food.

There has been no significant contamination of the environment by thallium from industry, unlike that caused by its neighbors in the periodic table, mercury and lead.

Thallium is very toxic to rats and is applied as a rat pesticide because of this. Thallium also has negative effects upon plants, such as colour changes in leaves and growth declines. Mammals, such as rabbits, are just as susceptible to the toxic effects of thallium as humans.