Thulium at a glance
- Atomic number
- 69
- Symbol
- Tm
- Atomic mass
- 168.93 u
- Category
- Lanthanide
- Period
- 6
- Electron configuration
- [Xe] 4f 13 6s 2
- State at 25 °C
- Solid
- Melting point
- 1545 °C
- Boiling point
- 1950 °C
- Density
- 9.321 g/cm³
- Electronegativity
- 1.25
- 1st ionization energy
- 597 kJ/mol
- Oxidation states
- +3
- Discovered
- 1879
Atomic structure
A neutral thulium atom has 69 protons and 69 electrons, and its most common isotope carries about 100 neutrons. One atom weighs about 2.81 × 10⁻²⁵ kg, so a mole of thulium (6.022 × 10²³ atoms) weighs 168.93 g. That nucleus holds 1.45 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 6 shells (K 2, L 8, M 18, N 31, O 8, P 2), which gives the configuration [Xe] 4f 13 6s 2. As a f-block element, both its outer s electrons and its inner f electrons can take part in bonding, which is why it shows more than one oxidation state. It has only one stable isotope, Tm-169. No other element has a stable isotope with mass number 169, so Tm-169 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) thulium is a solid. It melts at 1545 °C and boils at 1950 °C, so it stays liquid across a 405-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. Its density is 9.321 g/cm³, 9.32 times as dense as water. Its atoms have a radius of about 175 pm (Erbium next door is 175 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.25 on the Pauling scale. Pulling off its outermost electron takes 597 kJ/mol. A bond between thulium and chlorine would be mostly ionic, with an electronegativity difference of 1.9. It releases 99 kJ/mol when it gains an extra electron. Its common oxidation states are +3, so in compounds you will usually meet it as Tm³⁺. It shares much of its chemistry with the other lanthanides, especially its neighbours Holmium, Erbium, Ytterbium and Lutetium.
Where it comes from
It makes up 0.52 mg/kg of Earth's crust, the 62nd most abundant element there. It was first identified in 1879, making it the 69th element to be discovered (counting the 11 known since antiquity together).