Thallium (Tl)

Thallium properties

Atomic number

81

Atomic mass

204.38 u

Category

Post-transition metal

Group

13

Period

6

Block

p-block

State at room temp.

Solid

Electronegativity

1.62

Melting point

577 K (303.8 °C)

Boiling point

1,746 K (1,472.8 °C)

Density

11.85 g/cm³

Valence electrons

3

Common charges

+1, +3

Oxidation states

+3, +1

Atomic radius

196 pm

Ionization energy

6.108 eV

Electron affinity

0.2 eV

Radioactive

No, has stable isotopes

Discovered

1861

Thallium electron configuration

The electron configuration of Thallium (Tl) is [Xe] 4f14 5d10 6s2 6p1. Its 81 electrons are arranged in shells as 2, 8, 18, 32, 18, 3.

Protons, neutrons and electrons in Thallium

A neutral Thallium atom has 81 protons and 81 electrons. Most Thallium atoms have about 123 neutrons: the atomic mass rounded to 204, minus the atomic number 81.

Neutrons are approximate: atoms of the same element can have different numbers of neutrons (isotopes). Atomic number and mass

How many valence electrons does Thallium have?

3

Thallium has 3 valence electrons in its outermost shell, shell 6. These are the electrons involved in chemical bonding.

See every element: Periodic table with valence electrons

Thallium orbital diagram

Each box is an orbital that holds up to two electrons with opposite spins (↑↓). The inner shells are the same as the noble gas in brackets. How to write electron configurations

Molar mass of Thallium

204.38 g/mol

One mole of Thallium atoms (6.022 × 10²³ atoms) has a mass of 204.38 grams. Use this value to convert between grams and moles.

Thallium facts

• Thallium salts are tasteless and odorless, which made them a notorious poison in the 20th century.
• The blue pigment Prussian blue is used as an antidote for thallium poisoning because it binds thallium in the gut.
• Thallium (Tl) has atomic number 81 and is a solid at room temperature. It was discovered in 1861.

Position in the periodic table

Thallium is element 81, in period 6 and group 13 of the periodic table, in the p-block.

All post-transition metals

Uses of Thallium

Thallium is used only in small, specialized amounts because it is highly toxic. The radioactive isotope thallium-201 is injected during nuclear medicine stress tests to show how well blood reaches the heart muscle. Sodium iodide crystals doped with thallium are used to detect gamma rays in radiation detectors, and thallium compounds go into special infrared optics and high-refractive-index glass.

Who discovered Thallium?

William Crookes discovered thallium in England in 1861 when he saw a bright green line in the spectrum of residues from a sulfuric acid plant. Its name comes from the Greek thallos, meaning 'green shoot'.

Where does the name come from?

Thallium comes from the Greek thallos, meaning 'green shoot' or 'twig'. William Crookes discovered it in 1861 and named it after the brilliant green line in its spectrum, the color of fresh spring growth.

Discovery timeline