Thulium

Thulium (Tm) is a lanthanide with atomic number 69 and an atomic mass of 168.93. Its electron configuration is [Xe] 4f13 6s2.

Thulium key data

Symbol	Tm
Atomic number	69
Atomic mass (standard atomic weight)	168.93
Molar mass	168.93 g/mol
Protons	69
Neutrons (most abundant isotope, thulium-169)	100
Electrons (neutral atom)	69
Category	Lanthanide
Period	6
Group	f-block, no group
Block	f-block
Noble gas notation	[Xe] 4f13 6s2
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f13 5s2 5p6 6s2
Electrons per shell	2, 8, 18, 31, 8, 2

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of thulium

From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown.

Melting point	1818 K (1544.9 °C), 24th of 103 by melting point
Boiling point	2223 K (1949.9 °C), 54th of 93 by boiling point
Density	9.32 g/cm³, joint 29th of 96 by density
State at room temperature	Solid
Electronegativity	1.25 (Pauling), 74th of 95 by electronegativity
Van der Waals radius	227 pm, joint 30th of 99 by atomic radius
First ionisation energy	6.184 eV (596.7 kJ/mol), 70th of 102 by first ionisation energy
Common oxidation states	+3
Discovered	1879

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Thulium electron configuration

The full electron configuration of thulium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f13 5s2 5p6 6s2. With the noble-gas core collapsed it is [Xe] 4f13 6s2, and the outermost subshell is 6s2.

Bohr model of thulium (Tm): a nucleus and six shells holding 2, 8, 18, 31, 8 and 2 electrons from the inside out. — Bohr model of thulium (Tm). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of thulium (Tm), [Xe] 4f13 6s2, with the xenon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 4f. — Orbital diagram of thulium (Tm). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one thulium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 168.93 / (6.02214076 × 10²³)
= 2.8051 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 168.93
= 3.5649 × 10²¹ atoms

Thulium on the periodic table

Thulium is in period 6, outside the numbered groups, in the f-block.