Thulium (Tm)

Thulium properties

Atomic number

69

Atomic mass

168.93 u

Category

Lanthanide

Group

—

Period

6

Block

f-block

State at room temp.

Solid

Electronegativity

1.25

Melting point

1,818 K (1,544.8 °C)

Boiling point

2,223 K (1,949.8 °C)

Density

9.32 g/cm³

Valence electrons

2

Common charges

+3

Oxidation states

+3

Atomic radius

227 pm

Ionization energy

6.184 eV

Electron affinity

—

Radioactive

No, has stable isotopes

Discovered

1879

Thulium electron configuration

The electron configuration of Thulium (Tm) is [Xe] 4f13 6s2. Its 69 electrons are arranged in shells as 2, 8, 18, 31, 8, 2.

Protons, neutrons and electrons in Thulium

A neutral Thulium atom has 69 protons and 69 electrons. Most Thulium atoms have about 100 neutrons: the atomic mass rounded to 169, minus the atomic number 69.

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

How many valence electrons does Thulium have?

2

Thulium has 2 valence electrons in its outermost shell. As an f-block element, its 4f or 5f electrons can also take part in bonding.

See every element: Periodic table with valence electrons

Thulium 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 Thulium

168.93 g/mol

One mole of Thulium atoms (6.022 × 10²³ atoms) has a mass of 168.93 grams. Use this value to convert between grams and moles.

Thulium facts

• Thulium is the second-rarest lanthanide in Earth's crust, after radioactive promethium.
• It is named after Thule, an ancient name for a mysterious land in the far north, often linked with Scandinavia.
• Thulium (Tm) has atomic number 69 and is a solid at room temperature. It was discovered in 1879.

Position in the periodic table

Thulium is element 69, in period 6 of the periodic table. It is one of the Lanthanides in the f-block, shown in the rows below the main table.

All lanthanides

Uses of Thulium

Thulium is rare and expensive, so its uses are limited. Thulium fiber lasers are used in surgery, including treating kidney stones and enlarged prostates. When thulium-169 is irradiated in a nuclear reactor it becomes thulium-170, which serves as a compact radiation source in portable X-ray devices. Thulium is also studied in research on lasers and other advanced optical materials.

Who discovered Thulium?

Swedish chemist Per Teodor Cleve discovered thulium in 1879 in Uppsala, by separating it from impure erbium oxide.

Where does the name come from?

Thulium is named after Thule, the ancient Greek and Roman name for a mysterious land in the far north, often identified with Scandinavia. Per Teodor Cleve discovered and named the element in 1879.

Discovery timeline